Systems and processes for operating fuel cell systems

ABSTRACT

Processes and systems for operating molten carbonate fuel cell systems are described herein. A process for operating a molten carbonate fuel cell system includes providing a hydrogen-containing stream comprising molecular hydrogen to an anode portion of a molten carbonate fuel cell; controlling a flow rate of the hydrogen-containing stream to the anode such that molecular hydrogen utilization in the anode is less than 50%; mixing anode exhaust comprising molecular hydrogen from the molten carbonate fuel cell with a hydrocarbon stream comprising hydrocarbons, contacting at least a portion of the mixture of anode exhaust and the hydrocarbon stream with a catalyst to produce a steam reforming feed; separating at least a portion of molecular hydrogen from the steam reforming feed; and providing at least a portion of the separated molecular hydrogen to the molten carbonate fuel cell anode.

PRIORITY CLAIM

This application is a divisional application of U.S. application Ser.No. 13/009,233, filed Jan. 19, 2011, which is a continuation in part ofU.S. application Ser. No. 12/797,543, filed Jun. 9, 2010, which claimsthe benefit of U.S. Provisional Application No. 61/187,539, filed onJun. 16, 2009, which applications are hereby incorporated herein byreference.

FIELD OF THE INVENTION

The present invention relates to fuel cell systems and to processes ofoperating fuel cells. In particular, the present invention relates tosystems and processes of operating a molten carbonate fuel cell system.

BACKGROUND OF THE INVENTION

Molten carbonate fuel cells convert chemical energy into electricalenergy. Molten carbonate fuel cells are useful in that they deliver highquality reliable electrical power, are clean operating, and arerelatively compact power generators. These features make the use ofmolten carbonate fuel cells attractive as power sources in urban areas,shipping vessels, or remote areas with limited access to power supplies.

Molten carbonate fuel cells are formed of an anode, a cathode, and anelectrolytic layer sandwiched between the anode and cathode. Theelectrolyte includes alkali carbonate salts, alkaline-earth carbonatesalts, molten alkali carbonate salts, or mixtures thereof that may besuspended in a porous, insulating, and chemically inert matrix. Anoxidizable fuel gas, or a gas that may be reformed in the fuel cell toan oxidizable fuel gas, is fed to the anode. The oxidizable fuel gas fedto the anode is typically syngas—a mixture of oxidizable components,molecular hydrogen, carbon dioxide, and carbon monoxide. Anoxidant-containing gas, typically air and carbon dioxide, may be fed tothe cathode to provide the chemical reactants to produce carbonateanions. During operation of the fuel cell, the carbonate anions areconstantly renewed.

The molten carbonate fuel cell is operated at a high temperature,typically from 550° C. to 700° C., to react oxygen in theoxidant-containing gas with carbon dioxide to produce carbonate anions.The carbonate anions cross the electrolyte to interact with hydrogenand/or carbon monoxide from the fuel gas at the anode. Electrical poweris generated by the conversion of oxygen and carbon dioxide to carbonateions at the cathode and the chemical reaction of the carbonate ions withhydrogen and/or carbon monoxide at the anode. The following reactionsdescribe the electrical electrochemical reactions in the cell when nocarbon monoxide is present:

CO₂+0.5O₂+2e⁻→CO₃ ⁼  Cathode charge transfer:

CO₃ ⁼+H₂→H₂O+CO₂+2e⁻ and  Anode charge transfer:

H₂+0.5O₂→H₂O  Overall reaction:

If carbon monoxide is present in the fuel gas, the chemical reactionsbelow describe the electrochemical reactions in the cell.

CO₂+O₂+4e⁻→2CO₃ ⁼  Cathode charge transfer:

CO₃ ⁼+H₂→H₂O+CO₂+2e⁼ and CO₃ ⁼+CO→2CO₂+2e⁻  Anode charge transfer:

H₂+CO+O₂→H₂O+CO₂  Overall reaction:

An electrical load or storage device may be connected between the anodeand the cathode to allow electrical current to flow between the anodeand cathode. The electrical current powers the electrical load orprovides electrical power to the storage device.

Fuel gas is typically supplied to the anode by a steam reformer thatreforms a low molecular weight hydrocarbon and steam into hydrogen andcarbon oxides. Methane, for example, in natural gas, is a preferred lowmolecular weight hydrocarbon used to produce fuel gas for the fuel cell.Alternatively, the fuel cell anode may be designed to internally effecta steam reforming reaction on a low molecular weight hydrocarbon such asmethane and steam supplied to the anode of the fuel cell.

Methane steam reforming provides a fuel gas containing hydrogen andcarbon monoxide according to the following reaction: CH₄+H₂O⇄CO+3H₂.Typically, the steam reforming reaction is conducted at temperatureseffective to convert a substantial amount of methane and steam tohydrogen and carbon monoxide. Further hydrogen production may beeffected in a steam reformer by conversion of steam and carbon monoxideto hydrogen and carbon dioxide by a water-gas shift reaction of:H₂O+CO⇄CO₂+H₂.

In a conventionally operated steam reformer used to supply fuel gas to amolten carbonate fuel cell, however, little hydrogen is produced by thewater-gas shift reaction since the steam reformer is operated at atemperature that energetically favors the production of carbon monoxideand hydrogen by the steam reforming reaction. Operating at such atemperature disfavors the production of carbon dioxide and hydrogen bythe water-gas shift reaction.

Since carbon monoxide may be oxidized in the fuel cell to provideelectrical energy while carbon dioxide cannot, conducting the reformingreaction at temperatures favoring the reformation of hydrocarbons andsteam to hydrogen and carbon monoxide is typically accepted as apreferred method of providing fuel for the fuel cell. The fuel gastypically supplied to the anode by steam reforming, either externally orinternally, therefore, contains hydrogen, carbon monoxide, and smallamounts of carbon dioxide, unreacted methane, and water as steam.

Fuel gases containing non-hydrogen compounds such as carbon monoxide,however, are less efficient for producing electrical power in a moltencarbonate fuel cell than more pure hydrogen fuel gas streams. At a giventemperature, the electrical power that may be generated in a moltencarbonate fuel cell increases with increasing hydrogen concentration.This is due to the electrochemical oxidation potential of molecularhydrogen relative to other compounds. For example, Watanabe et al.describe, in “Applicability of molten carbonate fuel cells to variousfuels,” Journal of Power Sources, 2006, pp. 868-871, that a 10 kW moltencarbonate fuel cell stack operated at 90% fuel utilization and apressure of 0.49 MPa at a current density of 1500 A/m², with a 50%molecular hydrogen and 50% water feed produces an electrical powerdensity of 0.12 W/cm² at 0.792 volts while a 50% carbon monoxide and 50%water feed at the same operating conditions produces an electrical powerdensity of only 0.11 W/cm² at 0.763 volts. Therefore, fuel gas streamscontaining significant amounts of non-hydrogen compounds are not asefficient in producing electrical power in a molten carbonate fuel cellas fuel gases containing mostly hydrogen.

Molten carbonate fuel cells, however, are typically operatedcommercially in a “hydrogen-lean” mode, where the conditions of theproduction of the fuel gas, for example, by steam reforming, areselected to limit the amount of hydrogen exiting the fuel cell in thefuel gas. This is done to balance the electrical energy potential of thehydrogen in the fuel gas with the potential energy(electrochemical+thermal) lost by hydrogen leaving the cell withoutbeing converted to electrical energy.

Certain measures have been taken to recapture the energy of the hydrogenexiting the fuel cell, however, these are significantly less energyefficient than if the hydrogen were electrochemically reacted in thefuel cell. For example, the anode exhaust produced from theelectrochemically reacting fuel gas in the fuel cell has been combustedto drive a turbine expander to produce electricity. Doing so, however,is significantly less efficient than capturing the electrochemicalpotential of the hydrogen in the fuel cell since much of the thermalenergy is lost rather than converted by the expander to electricalenergy. Fuel gas exiting the fuel cell also has been combusted toprovide thermal energy for a variety of heat exchange applications.Almost 50% of the thermal energy, however, is lost in such heat exchangeapplications after combustion. Hydrogen is a very expensive gas to useto fire a burner utilized in inefficient energy recovery systems and,therefore, conventionally, the amount of hydrogen used in the moltencarbonate fuel cell is adjusted to utilize most of the hydrogen providedto the fuel cell to produce electrical power and minimize the amount ofhydrogen exiting the fuel cell in the fuel cell exhaust.

Other measures have been taken to produce more hydrogen from the fuelgas that is present in the anode exhaust and/or recycle hydrogen in theanode gas by providing the fuel gas to post reformers and/or gasseparation units. To recover the hydrogen and/or carbon dioxide, thefuel gas present in the anode is reformed in the post reformer to enrichthe anode gas stream in hydrogen and/or subjected to a water-gas shiftreaction to form hydrogen and carbon dioxide. Heat may be provided bythe anode gas stream.

Heat for inducing the methane steam reforming reaction in a steamreformer and/or converting liquid fuel into feed for the steam reformerhas also been provided by burners. Burners that combust anoxygen-containing gas with a fuel, typically a hydrocarbon fuel such asnatural gas, may be used to provide the required heat to the steamreformer. Flameless combustion has also been utilized to provide theheat for driving the steam reforming reaction, where the flamelesscombustion is also driven by providing a hydrocarbon fuel and an oxidantto a flameless combustor in relative amounts that avoid inducingflammable combustion. These methods for providing the heat necessary todrive steam reforming reactions and/or water-gas shift reactions arerelatively inefficient energetically since a significant amount ofthermal energy provided by combustion is not captured and is lost.

The hydrogen and carbon dioxide in the reformed gas stream may beseparated from the anode exhaust, for example, using pressure swingadsorption units and/or membrane separation units. The temperature ofthe anode exhaust is typically higher than the temperatures required bycommercial hydrogen and/or carbon dioxide separation units. The streammay be cooled, for example, through a heat exchanger, however, thermalenergy may be lost in the cooling process.

The separated hydrogen is fed to the anode portion of the fuel cell.Recycling the hydrogen to the anode may enrich the fuel gas entering themolten carbonate fuel cell with hydrogen. The separated carbon dioxideis fed to the cathode portion of the fuel cell. Recycling the carbondioxide to the cathode may enrich the air entering the molten carbonatefuel cell with carbon dioxide.

The cell potential (V) of a molten carbonate fuel cell is given by thedifference between the open circuit voltage (E) and the losses. For hightemperature fuel cells, activation losses are very small and the cellpotential may be obtained over the practical range of current densitiesby considering only ohmic losses. Thus cell potential V=E−iR, where Vand E have units of volts or millivolts, i is the current density(mA/cm²) and R is the total Ohmic resistance (Ωcm²), combiningelectrolyte, cathode and anode together. The open circuit voltage is thedominant term in the cell potential. The total voltage (electromotiveforce) for a molten carbonate fuel cell can be expressed using theNernst Equation, E=E°+(RT/2F)ln(P_(H2)P_(O2)^(0.5)/P_(H2O))+(RT/2F)ln(P_(CO2) ^(c)/P_(CO2) ^(a)), where E is thestandard cell potential, R is the universal gas constant of 8.314472JK⁻¹ mol⁻¹, T is the absolute temperature, and F is the Faraday constantof 9.64853399×104 C mol⁻¹. As shown, the cell voltage of a moltencarbonate fuel cell may be changed by varying the concentrations ofcarbon dioxide, hydrogen, and oxygen.

Certain measures have been taken to adjust the concentration ofhydrogen, oxygen, and carbon dioxide provided to the fuel cell tomaximize cell voltage. U.S. Pat. No. 7,097,925 (the '925 patent)maximizes the denominator of the ratio

$\frac{P_{H\; 2O\mspace{14mu} {({anode})}} \cdot P_{{CO}\; 2\mspace{14mu} {({anode})}}}{P_{H\; 2{({anode})}} \cdot P_{O\; 2\mspace{14mu} {({cathode})}}^{0.5} \cdot P_{{CO}\; 2\mspace{14mu} {({cathode})}}}$

by enriching streams fed to the anode of a molten carbonate fuel cellwith hydrogen while enriching streams fed to the cathode with oxygen andcarbon dioxide. The enriched streams are provided from pressure swingadsorption units.

While the prior art is effective in providing hydrogen, oxygen, andcarbon dioxide to the fuel cell at different concentrations, the processis relatively inefficient in producing the hydrogen, carbon dioxide, andoxygen streams. The process is also relatively thermally inefficient inproduction of the gases and thermal processes since the anode gas iscooled to remove the water prior to entering the pressure swingadsorption units. In addition, the reformers also do not convert aliquid hydrocarbon feedstock to a lower molecular weight feed for thesteam reformer, and insufficient heat is likely provided from the fuelcell to do so.

Further improvement in the efficiency in operating molten carbonate fuelcell systems for producing electricity and enhancing power density ofthe molten carbonate fuel cell is desirable.

SUMMARY OF THE INVENTION

The present invention is directed to a process for operating a moltencarbonate fuel cell, comprising:

providing a hydrogen-containing stream comprising molecular hydrogen toan anode portion of a molten carbonate fuel cell;

controlling a flow rate of the hydrogen-containing stream to the anodesuch that molecular hydrogen utilization in the anode is less than 50%;

mixing anode exhaust comprising molecular hydrogen from the moltencarbonate fuel cell with a hydrocarbon stream comprising hydrocarbons,wherein the anode exhaust mixed with the hydrocarbon stream has atemperature from 500° C. to 700° C.;

contacting at least a portion of the mixture of anode exhaust and thehydrocarbon stream with a catalyst to produce a steam reforming feedcomprising one or more gaseous hydrocarbons, molecular hydrogen, and atleast one carbon oxide;

separating at least a portion of the molecular hydrogen from the steamreforming feed; and

providing at least a portion of the separated molecular hydrogen to themolten carbonate fuel cell anode as at least a portion of thehydrogen-containing stream comprising molecular hydrogen.

In another aspect, the present invention is directed to a moltencarbonate fuel cell system, comprising:

a molten carbonate fuel cell configured to receive a hydrogen-containingstream comprising molecular hydrogen at a flow rate such that hydrogenutilization in an anode of the molten carbonate fuel cell is less than50%;

one or more reformers operatively coupled to the molten carbonate fuelcell, at least one reformer being configured to receive anode exhaustfrom the molten carbonate fuel cell and hydrocarbons, and beingconfigured to allow the anode exhaust to sufficiently mix withhydrocarbons to at least partially reform some of the hydrocarbons toproduce a reformed product stream, wherein the reformed product streamcomprises molecular hydrogen and at least one carbon oxide; and a hightemperature hydrogen-separation device that is part of, or coupled to,at least one of the reformers and operatively coupled to the moltencarbonate fuel cell, wherein the high temperature hydrogen-separationdevice comprises one or more high temperature hydrogen-separatingmembranes and is configured to receive a reformed product stream and toprovide a stream comprising at least a portion of the molecular hydrogento the molten carbonate fuel cell.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic drawing of an embodiment of a system that includesa first reformer and a high temperature hydrogen-separation device incombination with a second reformer for practicing a process describedherein.

FIG. 2 is a schematic drawing of an embodiment of a system that includesa first reformer with a heat exchanger, and a high temperaturehydrogen-separation device in combination with a second reformer forpracticing a process described herein.

FIG. 3 is a schematic drawing of an embodiment of a portion of thesystem in which the high temperature hydrogen-separation device islocated exterior of the second reformer.

FIG. 4 depicts cell voltage (mV) versus current density (mA/cm²) forembodiments of molten carbonate fuel cell systems operated at 1 bara.

FIG. 5 depicts power density (W/cm²) versus current density forembodiments of molten carbonate fuel cell systems operated at 1 bara.

FIG. 6 depicts cell voltage (mV) versus current density (mA/cm²) forvarious embodiments of molten carbonate fuel cell systems operated at 7bara.

FIG. 7 depicts power density (W/cm²) versus current density (mA/cm²) forembodiments of molten carbonate fuel cell systems operated at 7 bara.

FIG. 8 depicts percent hydrogen utilization vs. ΔP_(CO2) (bara) forembodiments of operating molten carbonate fuel cells using variousamounts of excess air at a given hydrogen utilization.

FIG. 9 depicts percent hydrogen utilization vs. ΔP_(CO2) (bara) forembodiments of operating molten carbonate fuel cells using methane orbenzene as a feed source.

FIG. 10 depicts cell voltage (mV) versus current density (mA/cm²) forembodiments of molten carbonate fuel cell systems using various fuelsources.

FIG. 11 depicts average excess carbon dioxide (ΔP_(CO2(avg))) versuspercent hydrogen utilization for embodiments of molten carbonate fuelcell systems using various fuel sources.

DETAILED DESCRIPTION OF THE INVENTION

The present invention described herein provides a highly efficientprocess for operating a molten carbonate fuel cell to generateelectricity at a high electrical power density and a system forperforming such a process. First, the process described herein maximizesthe electrical power density of the fuel cell system by minimizing,rather than maximizing, the per pass fuel utilization rate of the fuelin the molten carbonate fuel cell. The per pass fuel utilization rate isminimized to reduce the concentration of carbon dioxide and oxidationproducts, particularly water, throughout the anode path length of thefuel cell such that a high hydrogen concentration is maintainedthroughout the anode path length. A high electrical power density isprovided by the fuel cell since an excess of hydrogen is present forelectrochemical reaction at the anode electrode along the entire anodepath length of the fuel cell. In a process directed to achieving a highper pass fuel utilization rate, for example, greater than 60% fuelutilization, the concentration of oxidation products and carbon dioxidemay comprise greater than 40% of the fuel stream before the fuel hastraveled even halfway through the fuel cell. The concentration ofoxidation products and carbon dioxide may be several multiples of theconcentration of hydrogen in the fuel cell exhaust such that theelectrical power provided along the anode path may significantlydecrease as the fuel provided to the fuel cell progresses through theanode.

In the process described herein, the anode of a molten carbonate fuelcell is flooded with hydrogen over the entire path length of the anodesuch that the concentration of hydrogen at the anode electrode availablefor electrochemical reaction is maintained at a high level over theentire anode path length. Thus, the electrical power density of the fuelcell is maximized.

Use of a hydrogen-rich fuel that is primarily and preferably almost allhydrogen in the process maximizes the electrical power density of thefuel cell system since hydrogen has a significantly greaterelectrochemical potential than other oxidizable compounds typically usedin molten carbonate fuel cell systems (for example, carbon monoxide).

The process described herein produces a higher electrical power densityin a molten carbonate fuel cell system than systems disclosed in the artby utilizing a hydrogen-rich fuel and minimizing rather than maximizingthe per pass fuel utilization rate of the fuel cell. The minimization isachieved by separating and recycling hydrogen captured from the fuelexhaust, for example, anode exhaust, of the fuel cell and feeding thehydrogen from a feed and the recycle stream at selected rates tominimize the per pass fuel utilization.

The system described herein allows for a hydrogen rich stream to beprovided to the molten carbonate fuel cell while minimizing the amountof hydrocarbons provided to the fuel cell as compared to conventionalsystems. The system generates hydrogen rich streams that may be directlyintroduced into the anode portion of the molten carbonate fuel cell.

The system does not require a reformer directly coupled to the anodeand/or positioned in the anode of the molten carbonate fuel cell toensure sufficient hydrogen production as fuel for the anode of the fuelcell. Removing or eliminating a reformer or reforming zone in the moltencarbonate fuel cell allows the molten carbonate fuel cell to be floodedwith hydrogen while supplying a majority of the heat from the anodeexhaust to a first reformer. Fuel cells already equipped with internalreforming zones may be used in combination with the systems describedherein. Such fuel cells may be operated more economically and moreefficiently than systems disclosed in the art.

In the process described herein, the cathode of a molten carbonate fuelcell is flooded with carbon dioxide over the entire path length of thecathode such that the concentration of carbon dioxide at the cathodeelectrode available for electrochemical reaction is maintained at a highlevel over the entire cathode path length. Thus, the electrical powerdensity and/or cell voltage of the fuel cell is maximized.

The process described herein utilizes a carbon dioxide rich oxidant gascontaining stream, thus allowing operation of the fuel cell such thatthe carbon dioxide partial pressure in a majority of the cathode portionof the molten carbonate fuel cell is higher than a partial pressure ofcarbon dioxide in a majority of an anode portion of the molten carbonatefuel cell. Operating the fuel cell in this manner produces a higherelectrical power density than systems disclosed in the art.

Utilizing a carbon dioxide rich oxidant gas boosts voltage of the moltencarbonate fuel cell and inhibits carbon dioxide starvation of the moltencarbonate fuel cell. “Carbon dioxide starvation” refers to when thepartial pressure of carbon dioxide (P_(CO2) ^(c)) exiting the cathode isless than the partial pressure of carbon dioxide (P_(CO2) ^(a)) exitingthe anode. Providing excess carbon dioxide to the molten carbonate fuelcell at a minimum hydrogen utilization allows higher voltage and/orcurrent density to be obtained from the molten carbonate fuel cell.

The system described herein allows for a carbon dioxide rich stream tobe provided to the molten carbonate fuel cell from the hydrocarbonsprovided to the fuel cell, as compared to conventional systems. Carbondioxide generated from the system may be directly introduced into thecathode portion of the molten carbonate fuel cell. The system does notrequire an external source of carbon dioxide to ensure sufficient carbondioxide as feed for the cathode of the fuel cell.

The process described herein is also highly efficient since hydrogen andcarbon dioxide not utilized to produce electricity in the fuel cell arerecycled continuously through the fuel cell system. This enablesproduction of a high electrical power density relative to the lowestheating value of the fuel by eliminating the problem associated withlosing energy by hydrogen and/or carbon dioxide leaving the cell withoutbeing converted to electrical energy.

The system allows simultaneous feeding of an appropriate amount of airor molecular oxygen to the cathode of the fuel cell such that the carbondioxide to molecular oxygen molar ratio in the feed to the cathodeminimizes concentration polarization at the electrodes of the fuel cell.The system does not require oxygen enrichment of air. The process of theinvention allows simultaneous flooding of the anode with hydrogen andsimultaneous flooding of the cathode with carbon dioxide, whilecontrolling the quantity of molecular oxygen such that the carbondioxide to molecular oxygen molar ratio in the feed to the cathode is atleast 2 or at least 2.5.

Using the fuel cell system described in the invention allows the moltencarbonate fuel cell to be operated at 0.1 MPa (1 atm) at a high powerdensity. Typically, molten carbonate fuel cells are operated atpressures of from atmospheric to about 1 MPa (10 atm). Operating atpressures above atmospheric may affect the life span of seals in variousportions of the molten carbonate fuel cells. Operating the moltencarbonate fuel cell at or near atmospheric pressures may extend the lifespan of seals in the molten carbonate fuel cells while producingelectricity with high current densities for given cell voltages and/orpower densities.

In the process described herein, relatively little carbon dioxide isgenerated per unit of electricity produced by the process. The thermalintegration of a first reformer, a second reformer, and a hightemperature hydrogen-separation device with fuel cell, where the heatproduced in the fuel cell is transferred directly within the firstreformer by providing the hot anode exhaust stream from the fuel cell tothe first reformer, and subsequently feeding the product of the firstreformer directly within the second reformer, and then providing theproduct of the second reformer directly to the high temperaturehydrogen-separation device, reduces, and preferably eliminates,additional energy required to be provided to drive the endothermicreforming reactions in one or both reformers. Such thermal integrationreduces the need to provide additional energy, for example bycombustion. Thus, the amount of carbon dioxide produced in providingenergy to drive the reforming reaction(s) is reduced.

Recycling the anode exhaust stream through the system and provision of acarbon dioxide gas stream to the fuel cell, by separating the carbondioxide from the reformed gas product and then feeding the carbondioxide containing gas stream to the fuel cell, reduces the amount ofcarbon dioxide required to be produced by combustion. Such recyclingincreases the electrical efficiency of the process, and thereby reducesany carbon dioxide emissions.

Additionally, recycling the anode exhaust stream through the system andprovision of a hydrogen-containing gas stream rich in molecular hydrogento the fuel cell, by separating the hydrogen-containing gas stream fromthe reformed gas product, and then feeding the hydrogen-containing gasstream to the fuel cell, reduces the amount of hydrogen required to beproduced by the second reformer. Such recycling of anode exhaustincreases the electrical efficiency of the process. Furthermore, powerdensity of the molten carbonate fuel cell is improved, thus for the sameamount of power generated, fuel cells having smaller dimensions thanconventional fuel cells may be used to generate power.

The process described herein is more thermally and energeticallyefficient than processes disclosed in the art. Thermal energy from afuel cell exhaust is transferred directly into a first reformer. In someembodiments, a portion of the transferred thermal energy is subsequentlytransferred from the first reformer into a second reformer. The transferof thermal energy directly from the anode exhaust of the fuel cell tothe first reformer is highly efficient since the transfer is effected bymolecularly mixing a hot anode exhaust stream from the fuel celldirectly with a hydrocarbon stream comprising hydrocarbons and steam inthe first reformer. A hot feed is produced from the first reformer andsubsequently fed to the second reformer. The transfer of thermal energyfrom the first reformer to the second reformer is also highly efficientsince the thermal energy is contained in the feed fed from the firstreformer to the second reformer.

The process described herein is also more thermally efficient thanprocesses disclosed in the art since the heat from the anode exhaust isused to produce hydrogen at lower temperatures than typical steamreforming processes. In the process of the present invention, hydrogenmay be separated from the reformed product gases using a hightemperature hydrogen-separating device, where the high temperaturehydrogen-separating device is a membrane separation device. The hightemperature hydrogen-separation device may be operatively coupled to thesecond reformer such that the hydrogen may be separated from thereformed gases as the reforming reaction occurs in the second reformer.Separation of the hydrogen drives the equilibrium towards production ofhydrogen and lowers the temperature required to produce hydrogen.Further, more hydrogen may be produced at the lower reformingtemperatures since the equilibrium of the water-gas shift reaction(H₂O+CO⇄CO₂+H₂) favors the production of hydrogen at the lower reformingtemperatures, whereas it is not favored at conventional reformingreaction temperatures. A substantial amount, or all, of the molecularhydrogen and carbon dioxide produced from the second reformer isprovided to the molten carbonate fuel cell.

The process described herein allows liquid fuel to be utilized. Use ofliquid fuel allows one fuel to be used for more than one power source.For example, diesel fuel could be used on a vessel to power a moltencarbonate fuel cell and engines. Hydrogen is added to the first reformerthrough mixing of the anode exhaust with the liquid feed. Recycling ofthe hydrogen eliminates a need for a separate hydrogen source forthermal cracking of the liquid feed. Although some hydrogen is consumed,hydrogen is generated upon reformation of the cracked hydrocarbons. Theintegration of the reformers and high temperature hydrogen-separationdevice allows the system to generate substantially all the hydrogenneeded for the processes.

Reforming and/or hydrocracking of liquid fuels generates more carbondioxide per mole of hydrogen produced because the hydrogen to carbonratio is lower for fuels having a carbon number greater than 6 (forexample, diesel and naphtha) than for fuels having a carbon number lessthan 6 (for example, methane). Generation of more carbon dioxide permole of hydrogen produced allows substantially all, or all, of thecarbon dioxide needed for the molten carbonate fuel cell to be generatedfrom the liquid fuel. Generation of carbon dioxide in this manner mayeliminate or reduce the need to use a portion of the anode gas and/orfeed gas as a fuel for thermally inefficient combustion burners togenerate carbon dioxide. In the process described herein, excesshydrogen and carbon dioxide is produced which allows the hydrogen andcarbon dioxide to be recycled through the system.

The process of the invention allows the molten carbonate fuel cell to beoperated at pressures of at or less than 0.1 MPa (1 atm) and provides apower density of at least 0.12 W/cm² and/or a cell voltage of at least800 mV. In some embodiments, the process of the invention allows themolten carbonate fuel cell to be operated at pressures of, at, or lessthan 0.1 MPa (1 atm) and provides a power density of at least 0.12 W/cm²and/or a cell voltage of at least 800 mV.

As used herein, the term “hydrogen” refers to molecular hydrogen unlessspecified otherwise.

As used herein, the term “hydrogen source” refers to a compound fromwhich free hydrogen may be generated. For example, a hydrogen source maybe a hydrocarbon such as methane, or mixtures of such compounds, or ahydrocarbon containing mixture such as natural gas.

As used herein, when two or more elements are described as “operativelyconnected” or “operatively coupled,” the elements are defined to bedirectly or indirectly connected to allow direct or indirect fluid flowbetween the elements. The term “fluid flow,” as used herein, refers tothe flow of a gas or a fluid. As used in the definition of “operativelyconnected” or “operatively coupled” the term “indirect fluid flow” meansthat the flow of a fluid or a gas between two defined elements may bedirected through one or more additional elements to change one or moreaspects of the fluid or gas as the fluid or gas flows between the twodefined elements. Aspects of a fluid or a gas that may be changed inindirect fluid flow include physical characteristics, such as thetemperature or the pressure of a gas or a fluid, and/or the compositionof the gas or fluid, for example, by separating a component of the gasor fluid, or by condensing water from a gas stream containing steam.“Indirect fluid flow,” as defined herein, excludes changing thecomposition of the gas or fluid between the two defined elements bychemical reaction, for example, oxidation, or reduction of one or moreelements of the fluid or gas.

As used herein, the term “selectively permeable to hydrogen,” is definedas permeable to molecular hydrogen or elemental hydrogen and impermeableto other elements or compounds such that at most 10%, or at most 5%, orat most 1% of the non-hydrogen elements or compounds may permeate whatis permeable to the molecular or elemental hydrogen.

As used herein, the term “high temperature hydrogen-separation device,”is defined as a device or apparatus effective for separating hydrogen,in molecular or elemental form, from a gas stream at a temperature of atleast 250° C. (for example, at temperatures from 300° C. to 650° C.)

As used herein, “per pass hydrogen utilization” as referring to theutilization of hydrogen in a fuel in a molten carbonate fuel cell, isdefined as the amount of hydrogen in a fuel utilized to generateelectricity in one pass through the molten carbonate fuel cell relativeto the total amount of hydrogen in a fuel input into the fuel cell forthat pass. The per pass hydrogen utilization may be calculated bymeasuring the amount of hydrogen in a fuel fed to the anode of a fuelcell, measuring the amount of hydrogen in the anode exhaust of the fuelcell, subtracting the measured amount of hydrogen in the anode exhaustof the fuel cell from the measured amount of hydrogen in the fuel fed tothe fuel cell to determine the amount of hydrogen used in the fuel cell,and dividing the calculated amount of hydrogen used in the fuel cell bythe measured amount of hydrogen in the fuel fed to the fuel cell. Theper pass hydrogen utilization may be expressed as a percent bymultiplying the calculated per pass hydrogen utilization by 100.

As used herein, “excess carbon dioxide” refers a value for the partialpressure difference of carbon dioxide (ΔP_(CO2)) of the anode andcathode of the molten carbonate fuel cell. “Excess carbon dioxide”(ΔP_(CO2)) is calculated by measuring the partial pressure of the carbondioxide in the anode exhaust and the cathode exhaust at the anode andcathode outlets, respectively, and subtracting the measured carbondioxide partial pressure value for the anode from the measured carbondioxide partial pressure value for the cathode (e.g., ΔP_(CO2)=(P_(CO2)^(c))−(P_(CO2) ^(a))). For a counter current flow of feeds to the anodeand cathode, the “excess carbon dioxide” is calculated by measuring thepartial pressure of the carbon dioxide in the anode exhaust and thecathode exhaust at the anode outlet and cathode inlet and subtractingthe measured carbon dioxide partial pressure value for the anode fromthe measured value carbon dioxide partial pressure for the cathode(e.g., ΔP_(CO2)=(P_(CO2) ^(cinlet))−(P_(CO2) ^(aoutlet))).

Average excess carbon dioxide is calculated by the following equation.

□P _(CO2)(avg)=[{P _(CO2) ^(cinlet) +P _(CO2) ^(coutlet) }−{P _(CO2)^(ainlet)+P_(CO2) ^(aoutlet)}]/2

“Local excess carbon dioxide” refers a value for the partial pressuredifference of carbon dioxide (ΔP_(CO2(local))) of the molten carbonatefuel cell per percent of hydrogen utilization over a normalized distancewhere symmetry is assumed in the y direction (width). Local excesscarbon dioxide is calculated by ΔP_(CO2)(x)=(P_(CO2) ^(c))(x)−(P_(CO2)^(a))(x), where x is a normalized distance along the length of the anodecompartment.

FIGS. 1-3 depict schematics of embodiments of systems of the presentinvention for conducting processes in accordance with the presentinvention for operating a molten carbonate fuel cell to generateelectricity. Fuel cell system 10 includes molten carbonate fuel cell 12,first reformer 14, second reformer 16, high temperaturehydrogen-separation device 18, and oxidizing unit 20. In a preferredembodiment, second reformer 16, high temperature hydrogen-separationdevice 18, and oxidizing unit 20 are one unit. In a preferredembodiment, oxidizing unit 20 is a catalytic partial oxidation reformer.In an embodiment, high temperature hydrogen-separation device 18 is amolecular hydrogen membrane separation device. In an embodiment, secondreformer 16 includes a reforming zone, high temperaturehydrogen-separation device 18, catalytic partial oxidation reformer 20,and heat exchanger 22. The thermally integrated system providessufficient hydrogen and carbon dioxide for continuous operation of themolten carbonate fuel cell to generate electricity.

Molten carbonate fuel cell 12 includes anode 24, cathode 26, andelectrolyte 28. Electrolyte 28 is interposed between and contacts anode24 and cathode 26. Molten carbonate fuel cell 12 may be a conventionalmolten carbonate fuel cell and may, preferably, have a tubular or planarconfiguration. Molten carbonate fuel cell 12 may include a plurality ofindividual fuel cells stacked together. The individual fuel cells may bejoined electrically by interconnects and operatively connected such thatone or more gas streams may flow through the anodes of the stacked fuelcells and an oxidant-containing gas may flow through the cathodes of thestacked fuel cells. As used herein, the term “molten carbonate fuelcell” is defined as either a single molten carbonate fuel cell or aplurality of operatively connected or stacked molten carbonate fuelcells. Anode 24 of molten carbonate fuel cell 12 may be formed of poroussintered nickel compounds, nickel-chromium alloys, nickel withlithium-chromium oxide and/or nickel-copper alloys, or any materialsuitable for use as anodes for molten carbonate fuel cells. Cathode 26of molten carbonate fuel cell 12 may be formed of porous, sinteredmaterials such as nickel oxide, lithium-nickel-iron oxides, or anymaterial suitable for use as a cathode for molten carbonate fuel cells.

Gas streams are fed to the anode and cathode to provide the reactantsnecessary to generate electricity in fuel cell 12. Hydrogen-containingstreams enter anode 24 and oxidant-containing gas streams enter cathode26. Electrolyte section 28 is positioned in the fuel cell to preventhydrogen-containing gas stream(s) from entering the cathode and toprevent the oxidant-containing gas stream(s)—oxygen and carbon dioxidestreams—from entering the anode. Oxidant-containing gas stream(s)include one or more streams that contain oxygen and/or carbon dioxide.

Electrolyte section 28 conducts carbonate ions from the cathode to theanode for electrochemical reaction with oxidizable compounds in theanode gas stream such as hydrogen and, optionally, carbon monoxide atthe one or more anode electrodes. Electrolyte section 28 may be formedof molten salts of alkali metal carbonates, alkaline-earth metalcarbonates, or combinations thereof. Examples of electrolyte materialsinclude porous materials formed from lithium-sodium carbonate, lithiumcarbonate, sodium carbonate, lithium-sodium-barium carbonate,lithium-sodium-calcium carbonate, and lithium-potassium carbonate.

Fuel cell 12 is configured to allow hydrogen-containing gas stream(s) toflow from anode inlet 30 through anode 24 and out anode exhaust outlet32. The hydrogen-containing gas stream contacts one or more anodeelectrodes over the anode path length from the anode inlet 30 to theanode exhaust outlet 32.

In an embodiment, a gas stream containing molecular hydrogen,hereinafter, “a hydrogen-containing stream,” or a hydrogen source is fedthough line 34 to anode inlet 30. Metering valve 36 may be used toselect and control the flow rate of the hydrogen-containing stream toanode inlet 30. In a preferred embodiment, hydrogen is fed from hightemperature hydrogen-separation device 18, where the high temperaturehydrogen-separation device is a membrane unit, to anode 24 of fuel cell12 as described in detail below. In an embodiment, thehydrogen-containing gas stream may contain at least 0.6, or at least0.7, or at least 0.8, or at least 0.9, or at least 0.95, or at least0.98 mole fraction hydrogen.

A gas fed to the cathode includes an oxidant. As referred to herein,“oxidant” refers to a compound capable of being reduced by interactionwith molecular hydrogen. In some embodiments, the oxidant-containing gasfed to the cathode includes oxygen, carbon dioxide, inert gases, ormixtures thereof. In an embodiment, the oxidant-containing gas is acombination of an oxygen-containing gas stream and a carbon dioxidecontaining gas stream, or an oxygen/carbon dioxide-containing stream. Ina preferred embodiment, the oxidant-containing gas fed to the cathode isair or oxygen enriched air that has been blended with enough carbondioxide such that the molar ratio of carbon dioxide to oxygen is atleast 2 or at least 2.5.

An oxidant-containing gas may flow from cathode inlet 38 through cathode26 and then out through cathode exhaust outlet 40. Theoxidant-containing gas contacts one or more cathode electrodes over thecathode path length from cathode inlet 38 to cathode exhaust outlet 40.In one embodiment, an oxidant-containing gas may flow counter-current tothe flow of a hydrogen-containing gas flowing to anode 24 of fuel cell12.

In an embodiment, the oxidant-containing gas stream is fed fromoxidant-containing gas source 42 to cathode inlet 38 through line 44.Metering valve 46 may be used to select and control the rate the gasstream is fed to cathode 26. In some embodiments, the oxidant-containinggas is provided by an air compressor. The oxidant-containing gas streammay be air. In one embodiment, the oxidant-containing gas may be pureoxygen. In an embodiment, the oxidant-containing gas stream may beoxygen and/or carbon dioxide enriched air containing at least 13% byweight oxygen and/or at least 26% by weight carbon dioxide. In apreferred embodiment, the flow of air and/or carbon dioxide iscontrolled such that a molar ratio of carbon dioxide to molecular oxygenin the air is at least 2 or at least 2.5.

In one embodiment, the oxidant-containing gas stream is provided by acarbon dioxide containing gas stream and an oxygen-containing gasstream. The carbon dioxide stream and the oxygen-containing gas streammay come from two separate sources. In a preferred embodiment, amajority or substantially all of the carbon dioxide for molten carbonatefuel cell 12 is derived from the hydrocarbon stream comprisinghydrocarbons provided to first reformer 14. The carbon dioxidecontaining gas stream is fed from a carbon dioxide source to cathodeinlet 38 through line 44. The carbon dioxide containing gas streamprovided to fuel cell 12 may be fed to the same cathode inlet 38 as theoxygen-containing gas stream, or may be mixed with an oxygen-containinggas stream prior to being fed to cathode inlet 38. Alternatively, thecarbon dioxide containing gas stream may be provided to cathode 26through a separate cathode inlet.

In a preferred embodiment, the carbon dioxide stream is provided tocathode 26 of fuel cell 12 from high temperature hydrogen-separationdevice 18 via lines 48 and 44 as described herein. Oxygen may beprovided to cathode 26 of fuel cell 12 via line 44.

Gases fed to the cathode and/or anode, whether one stream or multiplestreams, may be heated in heat exchanger 22 or other heat exchangersprior to being fed to cathode 26 and/or anode 24, preferably byexchanging heat with an oxygen-depleted cathode exhaust stream exitingcathode exhaust 40 and connected to heat exchanger 22 through line 50.

In the process of the invention, the hydrogen-containing gas stream(s)are mixed with an oxidant at one or more of the anode electrodes ofmolten carbonate fuel cell 12 to generate electricity. The oxidant ispreferably carbonate derived from the reaction of carbon dioxide andoxygen flowing through cathode 26 and conducted across the electrolyteof the fuel cell. The hydrogen-containing gas stream and the oxidant aremixed in the anode at the one or more anode electrodes of fuel cell 12by feeding the hydrogen-containing gas stream and/or theoxidant-containing gas stream to the fuel cell at selected independentrates, as discussed in further detail below. The hydrogen-containing gasstream and the oxidant are preferably mixed at the one or more anodeelectrodes of the fuel cell to generate electricity at an electricalpower density of at least 0.1 W/cm², or at least 0.15 W/cm², or at least0.2 W/cm², or at least 0.3 W/cm², or at least 0.6 W/cm² at 1 bara.Higher power densities may be obtained at higher pressures and/or byusing enriched oxidant-containing gas streams (for example, enrichedair).

Molten carbonate fuel cell 12 is operated at a temperature effective toenable carbonate to traverse the electrolyte portion 28 from cathode 26to anode 24. Molten carbonate fuel cell 12 may be operated at atemperature from 550° C. to 700° C. or from 600° C. to 650° C. Theoxidation of hydrogen with carbonate at the one or more anode electrodesis an exothermic reaction. The heat of reaction generates the heatrequired to operate molten carbonate fuel cell 12. The temperature atwhich the molten carbonate fuel cell is operated may be controlled byseveral factors, including, but not limited to, regulating the feedtemperatures and feed flow of the hydrogen-containing gas and theoxidant-containing gas. Since hydrogen utilization is minimized, theexcess hydrogen is fed to the system and un-reacted hydrogen canpartially cool the molten carbonate fuel cell, by carrying excess heatto the first reformer. Adjusting the flow of the carbon dioxide streamand/or oxidant-containing stream to maintain the molar ratio of carbondioxide to molecular oxygen at about 2 requires enoughoxidant-containing gas to achieve an excess of molecular oxygen of about1.3 to 2.0 times the quantity need to react with the portion of thehydrogen utilized in the anode. Thus, the excess of oxygen depleted airor oxidant-containing gas, which exits in the cathode exhaust, may carrysignificant heat from the molten carbonate fuel cell. The temperature ofa hydrogen-containing stream described below provided to anode 24 ofmolten carbonate fuel cell 12 from the high temperaturehydrogen-separation device 18 may be reduced by heat recovery (forexample, through heat exchanger 22) prior to being provided to themolten carbon fuel cell anode. The temperature of a high-pressure carbondioxide stream described below provided to cathode 26 of moltencarbonate fuel cell 12 from the high temperature hydrogen-separationdevice 18 may be reduced by heat recovery (for example, through heatexchanger 22) prior to being provided to the molten carbon fuel cellcathode. The temperature of a effluent stream produced from catalyticpartial oxidation reformer 20 may be reduced by heat recovery (forexample, through heat exchanger 22) prior to being provided to themolten carbon fuel cell cathode. Waste heat from the fuel cell may beused to heat one or more of the streams utilized in the system. Ifnecessary, any supplemental systems for cooling molten carbonate fuelsknown in the art may be used to control the temperature of the moltencarbonate fuel cell.

In an embodiment, the oxidant-containing gas stream(s) fed to thecathode may be heated to a temperature of at least 150° C. or from 150°C. to 350° C. prior to being fed to cathode 26. In an embodiment, whenan oxygen-containing gas is used, the temperature of anoxygen-containing gas stream is controlled to a temperature from 150° C.to 350° C.

To initiate operation of fuel cell 12, the fuel cell is heated to itsoperating temperature—a temperature sufficient to melt the electrolytesalts to allow flow of carbonate ions. As shown in FIG. 1, operation ofmolten carbonate fuel cell 12 may be initiated by generating ahydrogen-containing gas stream in catalytic partial oxidation reformer20 and feeding the hydrogen-containing gas stream through lines 52 and34 to anode 24 of the molten carbonate fuel cell.

A hydrogen-containing gas stream may be generated in catalytic partialoxidation reformer 20 by combusting a portion of a hydrocarbon streamcomprising hydrocarbons described below, or a different hydrocarbonstream, for example, a fuel stream enriched in natural gas, and anoxidant-containing gas in catalytic partial oxidation reformer 20 in thepresence of a conventional partial oxidation catalyst, where an amountof oxygen in the oxidant-containing gas fed to catalytic partialoxidation reformer 20 is sub-stoichiometric relative to an amount ofhydrocarbons in the hydrocarbon stream. The flow of thehydrogen-containing gas stream may be controlled by valve 60.

As shown in FIG. 2, the fuel cell is heated to its operating temperatureby generating the hydrogen-containing gas stream in oxidizing unit 20and feeding the hydrogen-containing gas stream through lines 96, 104,and 34 to anode 24 of the molten carbonate fuel cell. The rate at whichthe hydrogen-containing gas stream from oxidizing unit 20 is fed toanode 24 via lines 96, 104, is controlled by three-way valve 102. Aportion of the heat from hydrogen-containing gas stream may be passedthrough heat exchanger 98 via line 96 to provide heat to first reformer14 and/or the hydrocarbon stream comprising hydrocarbons entering thefirst reformer.

Referring to FIGS. 1 and 2, the fuel fed to catalytic partial oxidationreformer 20 may be a liquid or gaseous hydrocarbon or mixtures ofhydrocarbons, and preferably is the same as the hydrocarbon streamcomprising hydrocarbons provided to first reformer 14. The fuel may befed to catalytic partial oxidation reformer 20 via line 62. In anembodiment, fuel fed to catalytic partial oxidation reformer 20 isenriched in natural gas and/or hydrogen from hydrogen source 64.

The oxidant fed to catalytic partial oxidation reformer 20 may be pureoxygen, air, or oxygen enriched air, hereinafter, “oxidant-containinggas.” Preferably, the oxidant-containing gas is air. The oxidant shouldbe provided to the catalytic partial oxidation reformer 20 such that anamount of oxygen in the oxidant is in sub-stoichiometric amountsrelative to the hydrocarbons fed to the catalytic partial oxidationreforming. In a preferred embodiment, the oxidant-containing gas is fedto catalytic partial oxidation reformer 20 through line 56 from oxidantsource 42. Valve 58 may control the rate at which oxidant-containing gas(air) is fed to catalytic partial oxidation reformer 20 and/or cathode26 of fuel cell 12. In an embodiment, the oxidant-containing gasentering catalytic partial oxidation reformer 20 may be heated byexchanging heat with an oxygen-depleted cathode exhaust stream exitingcathode exhaust 40.

In catalytic partial oxidation reformer 20, a hydrogen-containing gasstream is formed by combusting the hydrocarbons and oxidant in thepresence of a conventional partial oxidation catalyst, where the oxidantis in a sub-stoichiometric amount relative to the hydrocarbons. Thehydrogen-containing gas stream formed by contact of the hydrocarbons andthe oxidant in catalytic partial oxidation reformer 20 containscompounds that may be oxidized in fuel cell anode 24 by contact withcarbonate ions at one or more of the anode electrodes. Thehydrogen-containing gas stream from catalytic partial oxidation reformer20 preferably does not contain compounds that oxidize the one or moreanode electrodes in anode 24 of fuel cell 12.

The hydrogen-containing gas stream formed in catalytic partial oxidationreformer 20 is hot, and may have a temperature of at least 700° C., orfrom 700° C. to 1100° C., or from 800° C. to 1000° C. Use of the hothydrogen gas stream from catalytic partial oxidation reformer 20 toinitiate start up of molten carbonate fuel cell 12 is preferred in theprocess of the invention since it enables the temperature of the fuelcell to be raised to the operating temperature of the fuel cell almostinstantaneously. In an embodiment, heat may be exchanged in heatexchanger 22 between the hot hydrogen-containing gas from catalyticpartial oxidation reformer 20 and an oxidant-containing gas fed tocathode 26 when initiating operation of the fuel cell.

Referring to FIG. 1, the flow of the hot hydrogen-containing gas streamfrom the catalytic partial oxidation reformer 20 into fuel cell 12 maybe adjusted using valve 60, while feeding the hydrogen-containing gasstream into the anode 24 by opening valve 36. Valve 60 may be closedafter flow of a hydrogen-containing gas stream from high temperaturehydrogen-separation device 18 is initiated while decreasing or stoppingthe flow of hydrocarbon feed through line 62 and oxidant feed throughline 56 to catalytic partial oxidation reformer 20.

Referring to FIG. 2, the flow of the hot hydrogen-containing gas streamfrom the catalytic partial oxidation reformer 20 into fuel cell 12 byway of line 96 may be adjusted using three-way metering valve 102, whilefeeding the hydrogen-containing gas stream into the anode 24 by openingvalve 36. Valve 102 may be closed after generating a hydrogen-containinggas stream from high temperature hydrogen-separation device 18 whiledecreasing or stopping the flow of hydrocarbon feed through line 62 andoxidant feed through line 56 to catalytic partial oxidation reformer 20.Continuous operation of the fuel cell may then be conducted according tothe process of the invention.

Three-way metering valve 102 controls the flow of effluent fromcatalytic partial oxidation reformer 20 to anode 24 or cathode 26.During start-up, effluent from catalytic partial oxidation reformer 20is rich in hydrogen so the effluent is directed to anode 24 via line 104after passing through heat exchanger 98 via line 96. After start-up isinitiated and if catalytic partial oxidation reformer 20 is used toproduce carbon dioxide for cathode 26, metering valve 102 controls theflow of effluent from catalytic partial oxidation reformer 20 to cathode26 via line 96.

In another embodiment, operation of the fuel cell may be initiated witha hydrogen start-up gas stream from hydrogen source 64 that may bepassed through a start-up heater (not shown) to bring the fuel cell upto its operating temperature prior to introducing thehydrogen-containing gas stream via line 66 into fuel cell 12, as shownin FIG. 1. Hydrogen source 64 may be a storage tank capable of receivinghydrogen from the high temperature hydrogen-separation device 18. Thehydrogen source may be operatively connected to the fuel cell to permitintroduction of the hydrogen start-up gas stream into the anode of themolten carbonate fuel cell. The start-up heater may indirectly heat thehydrogen start-up gas stream to a temperature from 750° C. to 1000° C.Alternatively, the start-up heater may provide hydrogen by incompleteburning of the hydrogen from hydrogen source 64 provided to the heater.The start-up heater may be an electrical heater or may be a combustionheater. Upon reaching the operating temperature of the fuel cell, theflow of the hydrogen start-up gas stream into the fuel cell may be shutoff by a valve, and the hydrogen-containing gas stream may be introducedinto the fuel cell by opening a valve from the hydrogen generator to theanode of the fuel cell to start the operation of the fuel cell.

In one embodiment, first reformer 14 includes a catalytic partialoxidation reformer that is used to provide hydrogen to the moltencarbonate fuel cell on start-up. First reformer 14 may include one ormore catalyst beds that allow the first reformer to be used forautothermal reforming and then for steam reforming once the moltencarbonate fuel cell has reached operating temperature.

Once fuel cell 12 has started operating, cathode 26 and anode 24 emitexhaust. Exhaust from cathode 26 and anode 24 is hot and the heat fromthe exhaust may be thermally integrated with other units to produce athermally integrated system that produces all the fuel (hydrogen) andoxidant (carbonate ion) necessary for the operation of the fuel cell.

As shown in FIGS. 1 and 2, the processes described herein utilize asystem that includes thermally integrated hydrogen-separation separationdevice 18, molten carbonate fuel cell 12, first reformer 14, and secondreformer 16 and, in some embodiments, catalytic partial oxidizingreformer 20. High temperature hydrogen-separation device 18 comprisesone or more high temperature hydrogen-separating membranes 68 and isoperatively coupled to molten carbonate fuel cell 12. High temperaturehydrogen-separation device 18 provides a hydrogen-containing gas streamcontaining primarily molecular hydrogen to anode 24 of fuel cell 12,while the exhaust from the anode of molten carbonate fuel cell 12 isprovided to first reformer 14. First reformer 14 and second reformer 16may be one unit or two units operatively coupled. First reformer 14 andsecond reformer 16 may include one or more reforming zones. In anembodiment, first reformer 14 and second reformer 16 are one unit thatincludes a first reforming zone and a second reforming zone.

A hydrocarbon stream comprising hydrocarbons is provided to firstreformer 14 via line 62 and the anode exhaust is mixed with thehydrocarbons. The process is thermally integrated, where heat to drivethe endothermic reforming reactions in first reformer 14 is providedfrom the anode exhaust of the exothermic molten carbonate fuel cell 12directly within the first reformer and/or with the hydrocarbons in thehydrocarbon stream provided to the first reformer. In an embodiment, aportion of the heat from the anode exhaust is mixed with thehydrocarbons in a heat exchanger in or operatively coupled to the firstreformer. As shown in FIG. 2, additional heat to first reformer 14 maybe provided from a hot effluent stream from catalytic partial oxidationreformer 20. In first reformer 14, at least a portion of thehydrocarbons from the hydrocarbon stream are cracked and/or reformed toproduce a feed stream that is provided to second reformer 16 via line70.

Second reformer 16 is operatively coupled to high temperaturehydrogen-separation device 18 and the high temperaturehydrogen-separation device produces at least a portion, a majority, atleast 75% by volume, or at least 90% by volume, or substantially all ofthe hydrogen-containing gas that enters anode 24 of molten carbonatefuel cell 12. High temperature hydrogen-separation device may bepositioned after second reformer 16 and before molten carbonate fuelcell 12. In a preferred embodiment, high temperature hydrogen-separationdevice 18 is a membrane separation unit that is part of second reformer16. The high temperature hydrogen-separation device 18 separateshydrogen from the reformed product. The separated hydrogen is providedto anode 24 of molten carbonate fuel cell 12.

In an embodiment of the process, the hydrocarbon stream contains one ormore of any vaporizable hydrocarbon that is liquid at 20° C. atatmospheric pressure (optionally oxygenated) that is vaporizable attemperatures up to 400° C. at atmospheric pressure. Such hydrocarbonsmay include, but are not limited to, petroleum fractions such asnaphtha, diesel, jet fuel, gas oil, and kerosene having a boiling pointrange of 50° C. to 360° C. In an embodiment, the hydrocarbon stream isdecane. In a preferred embodiment, the hydrocarbon stream is dieselfuel. In an embodiment, the hydrocarbon stream contains hydrocarbonshaving a carbon number ranging from five to twenty-five. In a preferredembodiment, the hydrocarbon stream contains at least 0.5, or at least0.6, or at least 0.7, or at least 0.8 mole fraction of hydrocarbonscontaining at least five, or at least six, or at least seven carbonatoms.

The hydrocarbon stream may optionally contain some hydrocarbons that aregaseous at 25° C. such as methane, ethane, propane, or other compoundscontaining from one to four carbon atoms that are gaseous at 25° C. Thehydrocarbon stream may be treated prior to being fed to first reformer14 and/or heated in heat exchanger 72 to remove any materials that maypoison any catalyst used in the first reformer for the conversion ofhigher molecular weight hydrocarbons to lower molecular weighthydrocarbons. For example, the hydrocarbon stream may have undergone aseries of treatments to remove metals, sulfur, and/or nitrogencompounds.

In an embodiment of the process, the hydrocarbon stream is mixed withnatural gas that contains at least 20% by volume, or at least 50% byvolume, or at least 80% by volume carbon dioxide. If necessary, thenatural gas has been treated to remove hydrogen sulfide. In anembodiment, a hydrocarbon stream that has at least 20% by volume ofcarbon dioxide, at least 50% by volume carbon dioxide, or at least 70%by volume of carbon dioxide may be used as a fuel source.

In an embodiment, the hydrocarbon stream may be provided to firstreformer 14 at a temperature of at least 150° C., preferably from 200°C. to 400° C., where the hydrocarbon stream may be heated to a desiredtemperature in heat exchangers as described below. The temperature thatthe hydrocarbon stream is fed to first reformer 14 may be selected to beas high as possible to vaporize the hydrocarbons without producing coke.The temperature of the hydrocarbon stream may range from 150° C. to 400°C. Alternatively, but less preferred, the hydrocarbon stream may be feddirectly to first reformer 14 at a temperature of less than 150° C., forexample without heating the hydrocarbon stream, provided the sulfurcontent of the hydrocarbon stream is low.

As shown in FIG. 1, the hydrocarbon stream may be passed through one ormore heat exchangers 72 to heat the feed. The hydrocarbon stream, may beheated by exchanging heat with cathode exhaust stream separated fromcathode 26 of molten carbonate fuel cell 12 and fed to heat exchanger 72via line 74. The rate at which the cathode exhaust stream is fed to heatexchangers 72 and 22 may be controlled by adjusting metering valves 76and 78.

In a preferred embodiment, separated anode exhaust stream is fed intoone or more reforming zones of first reformer 14 via line 80. The rateat which the anode exhaust stream is fed to the first reformer 14 may becontrolled by adjusting metering valve 82. The temperature of the anodeexhaust may range from about 500° C. to about 700° C., and preferably isabout 650° C.

The anode exhaust stream includes hydrogen, steam, and reaction productsfrom the oxidation of fuel fed to anode 24 of fuel cell 12 and unreactedfuel. In an embodiment, the anode exhaust stream contains at least 0.5,or at least 0.6, or at least 0.7 mole fraction hydrogen. The hydrogen inthe anode exhaust stream fed to first reformer 14 or a reforming zone ofthe first reformer may help prevent the formation of coke in the firstreformer. In an embodiment, the anode exhaust stream contains from0.0001 to about 0.3, or from 0.001 to about 0.25, or from 0.01 to about0.2 mole fraction water (as steam). In addition to hydrogen, steampresent in the anode exhaust stream fed to first reformer 14 or areforming zone of the first reformer also may help prevent the formationof coke in the first reformer. The anode exhaust stream may containenough hydrogen to inhibit coking and enough steam to reform most of thehydrocarbons in the hydrocarbon stream to methane, hydrogen, and carbonmonoxide. Thus, less steam may be needed for reforming hydrocarbons inthe first reformer and/or second reformer.

Optionally, steam may be fed to first reformer 14 or a reforming zone ofthe first reformer via line 84 to be mixed with the hydrocarbon streamin the first reformer or the reforming zone of the first reformer. Steammay be fed to first reformer 14 or a reforming zone of the firstreformer to inhibit or prevent coke formation in the first reformer and,optionally, to be utilized in reforming reactions effected in the firstreformer. In an embodiment, steam is fed to first reformer 14 orreforming zone of the first reformer at a rate where the molar ratio oftotal steam added to the first reformer is at least twice, or at leastthree times, the moles of carbon in the hydrocarbon stream added to thefirst reformer. The total steam added to the first reformer may includesteam from the anode exhaust, steam from an external source, forexample, through line 84, or mixtures thereof. Providing a molar ratioof at least 2:1, or at least 2.5:1, or at least 3:1, or at least 3.5:1of steam to carbon in the hydrocarbon stream in first reformer 14 a orreforming zone of the first reformer may be useful to inhibit cokeformation in the first reformer. Metering valve 86 may be used tocontrol the rate that steam is fed to first reformer 14 or a reformingzone of the first reformer through line 84. Since the anode exhaustincludes a significant amount of hydrogen, less coking tends to occurduring reforming. Thus, the amount of optional steam fed to firstreformer 14 may be significantly less than the amount of steam used forconventional reforming units.

Steam may be fed to first reformer 14 at a temperature of at least 125°C., preferably from 150° C. to 300° C., and may have a pressure from 0.1MPa to 0.5 MPa, preferably having a pressure equivalent to or below thepressure of the anode exhaust stream fed to the first reformer asdescribed herein. The steam may be generated by heating high-pressurewater, having a pressure of at least 1.0 MPa, preferably 1.5 MPa to 2.0MPa, by passing the high-pressure water via line 88 through heatexchanger 90. The high-pressure water is heated to form high-pressuresteam by exchanging heat with cathode exhaust fed after cathode exhaustfeed has passed through heat exchanger 72 via line 74. Alternatively,the cathode exhaust may be fed directly to heat exchanger 90 (not shown)or to one or more heat exchangers. Upon exiting heat exchanger 90 or thefinal heat exchanger if more than one heat exchanger is utilized, thehigh-pressure steam may then be fed to line 84 via line 92. Thehigh-pressure steam may be depressurized to the desired pressure byexpanding the high-pressure steam through an expander, then feeding toit to the first reformer. Alternatively, steam may be generated for usein the first reformer 14 by feeding low-pressure water through the oneor more heat exchangers 90 and passing the resulting steam into thefirst reformer.

Optionally, high-pressure steam that is not utilized in first reformer14 or second reformer 16 may be expanded through other power devicessuch as a turbine (not shown) together with any non-utilizedhigh-pressure carbon dioxide stream, or, optionally, without thehigh-pressure carbon dioxide stream. Power sources may be used togenerate electricity and/or in addition to electricity generated by thefuel cell 12. Power generated by the power sources and/or the fuel cellmay be used to power compressor 94 and/or any other compressors used inthe process of the invention.

The hydrocarbon stream, optional steam, and the anode exhaust stream aremixed and contacted with a reforming catalyst in first reformer 14 or areforming zone of the first reformer at a temperature effective tovaporize any hydrocarbons not in vapor form and to crack thehydrocarbons to form the feed.

The reforming catalyst may be a conventional reforming catalyst, and maybe any known catalyst in the art. Typical reforming catalysts, which canbe used include, but are not limited to, Group VIII transition metals,particularly nickel and a support or substrate that is inert under hightemperature reaction conditions. Suitable inert compounds for use as asupport for the high temperature reforming/hydrocracking catalystinclude, but are not limited to, α-alumina and zirconia.

In a preferred embodiment, the hydrocarbon stream, the anode exhaust,and optional steam are mixed and contacted with a catalyst at atemperature from about 500° C. to about 650° C. or from about 550° C. to600° C. with all the heat necessary for the reforming reaction suppliedby the anode exhaust. In an embodiment, the hydrocarbon stream, optionalsteam, and anode exhaust stream are mixed and contacted with a catalystat a temperature of at least 400° C., or in a range from 450° C. to 650°C., or from 500° C. to 600° C.

Heat supplied from the anode exhaust stream fed from the exothermicmolten carbonate fuel cell 12 to first reformer 14 or a reforming zoneof the first reformer drives the endothermic cracking and reformingreactions in the first reformer. The anode exhaust stream fed frommolten carbonate fuel cell 12 to first reformer 14 and/or a reformingzone of the first reformer is very hot, having a temperature of at least500° C., typically having a temperature from 550° C. to 700° C., or from600° C. to 650° C. The transfer of thermal energy from molten carbonatefuel cell 12 to first reformer 14 or a reforming zone of the firstreformer is extremely efficient since thermal energy from the fuel cellis contained in the anode exhaust stream, and is transferred to themixture of hydrocarbon stream, optional steam, and anode exhaust streamin first reformer 14 or a reforming zone of the first reformer bydirectly mixing the anode exhaust stream with the hydrocarbon stream andsteam.

In a preferred embodiment of the process described herein, the anodeexhaust stream provides at least 99%, or substantially all, of the heatrequired to produce the feed from the mixture of the hydrocarbon stream,the optional steam, and the anode exhaust stream. In a particularlypreferred embodiment, no heat source other than the anode exhaust streamis provided to first reformer 14 to convert the hydrocarbon stream tothe feed.

In an embodiment, the pressure at which the anode exhaust stream, thehydrocarbon stream, and the optional steam are contacted with thereforming catalyst in first reformer 14 may range from 0.07 MPa to 3.0MPa. If high-pressure steam is not fed to the first reformer 14, theanode exhaust stream, the hydrocarbon stream, and optional low-pressuresteam may be contacted with the reforming catalyst in the first reformerat a pressure at the low end of the range, typically from 0.07 MPa to0.5 MPa, or from 0.1 MPa to 0.3 MPa. If high-pressure steam is fed tofirst reformer 14, the anode exhaust stream, the hydrocarbon stream, andthe steam may be contacted with the reforming catalyst in at the higherend of the pressure range, typically from 1.0 MPa to 3.0 MPa, or from1.5 MPa to 2.0 MPa.

Referring to FIG. 2, first reformer 14 may be heated to temperatureshigher than 630° C. or from 650° C. to 900° C., or from 700° C. to 800°C. by exchanging heat with effluent from catalytic partial oxidationreformer 20 via line 96. Line 96 is operatively coupled to heatexchanger 98. Heat exchanger 98 may be a part of line 96. Heat exchanger98 may be in first reformer 14 or connected to the first reformer suchthat heat may be exchanged with the hydrocarbon stream entering thefirst reformer. The rate at which the effluent from catalytic partialoxidation reformer 20 is fed first reformer 14 may be controlled byadjusting metering valve 100 and three-way metering valve 102.

Contacting the hydrocarbon stream, steam, catalyst, and the anodeexhaust stream in first reformer 14 at a temperature of at least 500°C., or from 550° C. to 950° C., or from 600° C. to 800° C., or from 650°C. to 750° C., may crack and/or reform at least a portion of thehydrocarbons and form the feed. Cracking and/or reforming ofhydrocarbons in the hydrocarbon stream reduces the number of carbonatoms in hydrocarbon compounds in the hydrocarbon stream, therebyproducing hydrocarbon compounds having reduced molecular weight. In anembodiment, the hydrocarbon stream may comprise hydrocarbons containingat least 5, or at least 6, or at least 7 carbon atoms that are convertedto hydrocarbons useful as feed to second reformer 16 containing at most4, or at most 3, or at most 2 carbon atoms. In an embodiment, thehydrocarbons in the hydrocarbon stream may be reacted in first reformer14 or a reforming zone of the first reformer such that the feed producedfrom the first reformer may be comprised of not more than 0.1, or notmore than 0.05, or not more than 0.01 mole fraction of hydrocarbons withfour carbon atoms or more. In an embodiment, hydrocarbons in thehydrocarbon stream may be cracked and/or reformed such that at least0.7, or at least 0.8, or at least 0.9, or at least 0.95 mole fraction ofthe resulting hydrocarbons in the feed produced from the hydrocarbons inhydrocarbon stream is methane. In an embodiment, cracking and/orreforming of the hydrocarbons in the hydrocarbon stream produces a feedthat has an average carbon number of hydrocarbons in the feed is at most1.3, at most 1.2, or at most 1.1.

As noted above, hydrogen and steam from the anode exhaust stream andoptional steam added to first reformer 14 inhibit the formation of cokein the first reformer as hydrocarbons are cracked to form the feed. In apreferred embodiment, the relative rates that the anode exhaust stream,the hydrocarbon stream, and the steam are fed to first reformer 14 areselected so the hydrogen and steam in the anode exhaust stream and thesteam added to the first reformer via line 84 prevent the formation ofcoke in the first reformer.

In an embodiment, contacting the hydrocarbon stream, steam, and anodeexhaust with the reforming catalyst in first reformer 14 at atemperature of at least 500° C., or from 550° C. to 700° C., or from600° C. to 650° C., may also effect at least some reforming of thehydrocarbons in the hydrocarbon stream and feed produced within firstreformer 14 to produce hydrogen and carbon oxides, particularly carbonmonoxide. The amount of reforming may be substantial, where the feedresulting from both cracking and reforming in first reformer 14 orreforming zone of the first reformer may contain at least 0.05, or atleast 0.1, or at least 0.15 mole fraction carbon monoxide.

The temperature and pressure conditions in first reformer 14 or areforming zone of the first reformer may be selected so the feedproduced in the first reformer comprises light hydrocarbons that aregaseous at 20° C., typically containing 1 to 4 carbon atoms. In apreferred embodiment, the hydrocarbons in the feed produced by the firstreformer, hereinafter “steam reforming feed,” are comprised of at least0.6, or at least 0.7, or at least 0.8, or at least 0.9 mole fractionmethane. The steam reforming feed also comprises hydrogen from the anodeexhaust stream and, if further reforming is effected in the firstreformer, from reformed hydrocarbons. The steam reforming feed alsocomprises steam from the anode exhaust stream and, optionally, from thereformer steam feed. If substantial reforming is effected in firstreformer 14 or a reforming zone of the first reformer, the feed producedfrom the first reformer provided to second reformer 16 may comprisecarbon monoxide in addition to carbon dioxide.

In the process of the invention, the steam reforming feed is providedfrom first reformer 14 to second reformer 16, which is operativelyconnected to the first reformer through line 70. The steam reformingfeed exiting first reformer 14 may have a temperature of from 500° C. to650° C. or from 550° C. to 600° C. The temperature of the steamreforming feed exiting first reformer 14 may be lowered prior to beingfed to second reformer 16 by exchanging heat in one or more heatexchangers 90 prior to being fed to second reformer 16. Optionally, thesteam reforming feed is not cooled prior to entering the secondreformer. In embodiments when first reformer 14 is heated by othersources (for example as shown in FIG. 2, steam and/or heat fromcatalytic partial oxidation reformer 20) the feed exiting the firstreformer may have a temperature of from 650° C. to 950° C., or from 700°C. to 900° C., or from 750° C. to 800° C.

The steam reforming feed may be cooled by exchanging heat with water fedinto the system, cooling the feed, and producing steam that may be fedto the first reformer 14 as described above. If more than one heatexchanger 90 is utilized, the steam reforming feed and water/steam maybe fed in series to each of the heat exchanger, preferably in acountercurrent flow to cool the feed and to heat the water/steam. Thesteam reforming feed may be cooled to a temperature of from 150° C. to650° C., or from 150° C. to 300° C., or from 400° C. to 650° C., or from450° C. to 550° C.

The cooled steam reforming feed may be fed from heat exchanger 90 tocompressor 94, or, in another embodiment, may be fed directly to secondreformer 16. Alternatively, but less preferably, the steam reformingfeed exiting first reformer 14 or a reforming zone of the first reformermay be fed to compressor 94 or second reformer 16 without cooling.Compressor 94 is a compressor capable of operating at high temperatures,and preferably is a commercially available StarRotor compressor. Thesteam reforming feed may have a pressure of at least 0.5 MPa and atemperature from 400° C. to 800° C., preferably from 400° C. to 650° C.The steam reforming feed may be compressed by compressor 94 to apressure of at least 0.5 MPa, or at least 1.0 MPa, or at least 1.5 MPa,or at least 2 MPa, or at least 2.5 MPa, or at least 3 MPa, to maintainsufficient pressure in reforming zone 108 of second reformer 16. In anembodiment, the steam reforming feed is compressed to a pressure of from0.5 MPa to 6.0 MPa prior to providing the feed stream to the secondreformer.

The optionally compressed, optionally cooled steam reforming feedcomprising hydrogen, light hydrocarbons, steam, and, optionally, carbonmonoxide, is fed to second reformer 16. The steam reforming feed mayhave a pressure of at least 0.5 MPa and a temperature from 400° C. to800° C., preferably from 400° C. to 650° C. In an embodiment,temperature of the steam reforming feed produced from first reformer 14may be increased after exiting compressor 94, if necessary, bycirculating a portion of the feed through heat exchangers 90 and/or 72.

Optionally, additional steam may be added into reforming zone 108 of thesecond reformer 16 for mixing with the steam reforming feed produced bythe first reformer, if necessary for reforming the feed. In a preferredembodiment, the additional steam may be added by injecting high-pressurewater from the water inlet line 88 into compressor 94 through line 110for mixing with the feed as the feed is compressed in the compressor. Inan embodiment (not shown), high-pressure water may be injected into thefeed by mixing the high-pressure water and feed in heat exchanger 90. Inanother embodiment (not shown), high-pressure water may be injected intothe feed in line 110 either before or after passing the feed to heatexchanger 90 or before or after passing the feed to the compressor 94.In an embodiment, high-pressure water may be injected into line 70, orinto compressor 94 or in heat exchanger 90, where either the compressoror the heat exchanger are not included in the system.

The high-pressure water is heated to form steam by mixing with the steamreforming feed, and the steam reforming feed is cooled by mixing withthe water. The cooling provided to the steam reforming feed by the waterinjected therein may eliminate or reduce the need for heat exchanger 90preferably limiting the number of heat exchangers used to cool the steamreforming feed to at most one.

Alternatively, but less preferred, high-pressure steam may be injectedinto reforming zone 108 of second reformer 16 or into line 70 to thesecond reformer to be mixed with the steam reforming feed. Thehigh-pressure steam may be steam produced by heating high-pressure waterinjected into the system through water inlet line 88 in heat exchanger90 by exchanging heat with the feed exiting first reformer 14. Thehigh-pressure steam may be fed to second reformer 16 through line 112.Metering valve 114 may be used to control the flow of steam to thesecond reformer. The high-pressure steam may have a pressure similar tothat of the feed being fed to the second reformer. Alternatively, thehigh-pressure steam may be fed to line 70 to be mixed with the feedprior to the feed being fed to compressor 94 so the mixture of steam andfeed may be compressed together to a selected pressure. Thehigh-pressure steam may have a temperature from 200° C. to 500° C.

The rate at which the high-pressure water or high-pressure steam is fedinto the system may be selected and controlled to provide an amount ofsteam to first reformer 14 and/or second reformer 16 effective tooptimize reactions in the reformers to produce a hydrogen-containing gasstream. The rate at which steam, other than steam in the anode exhauststream, is provided to first reformer 14 may be controlled by adjustingmetering valves 116 and 118, which control the rate water is fed to thesystem, or by adjusting metering valves 86, 120, and 114, which controlthe rates at which steam is fed to first reformer 14 second reformer 16.Steam may be supplied to additional components in the system such as,for example, a turbine.

If high-pressure water is injected into second reformer 16, meteringvalves 114 and 120 may be adjusted to control the rate the water isinjected into the second reformer through line 112. If high-pressuresteam is injected into second reformer 16 or into line 70, meteringvalves 114, 116, and 118 may be adjusted to control the rate the steamis injected into second reformer 16 or into line 70. The flow of steammay be adjusted to provide a molar ratio of at least 2:1, or at least2.5:1, or at least 3:1, or at least 3.5:1 of steam to carbon.

The steam reforming feed produced by the first reformer and, optionally,additional steam are fed into reforming zone 108 of second reformer 16.The reforming zone may, and preferably does, contain a reformingcatalyst therein. The reforming catalyst may be a conventional steamreforming catalyst, and may be known in the art. Typical steam reformingcatalysts, which can be used include, but are not limited to, Group VIIItransition metals, particularly nickel. It is often desirable to supportthe reforming catalysts on a refractory substrate (or support). Thesupport, if used, is preferably an inert compound. Suitable inertcompounds for use as a support contain elements of Group III and IV ofthe Periodic Table, such as, for example the oxides or carbides of Al,Si, Ti, Mg, Ce, and Zr.

The steam reforming feed and, optionally additional steam, are mixed andcontacted with the reforming catalyst in the reforming zone 108 at atemperature effective to form a reformed product gas containing hydrogenand carbon oxides. The reformed product gas may be formed by steamreforming the hydrocarbons in the feed. The reformed product gas mayalso be formed by water-gas shift reacting steam and carbon monoxide inthe feed and/or produced by steam reforming the feed. In an embodiment,second reformer 16 may act more as a water-gas shift reactor if asubstantial amount of reforming was effected in first reformer 14 or areforming zone of the first reformer and the steam reforming feedcontains substantial amounts of carbon monoxide. The reformed productgas comprises hydrogen and at least one carbon oxide. In an embodiment,the reformed product gas comprises gaseous hydrocarbons, hydrogen and atleast one carbon oxide. Carbon oxides that may be in the reformedproduct gas include carbon monoxide and carbon dioxide.

In an embodiment, heat from effluent from catalytic partial oxidationreformer 20 may be heat exchanged with the steam reforming feed streambeing provided to and/or in reforming zone 108. A temperature of theeffluent from catalytic partial oxidation reformer 20 may range from750° C. to 1050° C., or from 800° C. to 1000° C., or from 850° C. to900° C. Heat from the effluent may heat reforming zone 108 of secondreformer 16 to a temperature from about 500° C. to about 850° C., orfrom about 550° C. to 700° C. A temperature in reforming zone 108 ofsecond reformer 16 may be sufficient to reform substantially all, orall, of the feed from first reformer 14 to produce a reformed productgas that comprises hydrogen and at least one carbon oxide.

The reformed product gas may enter high temperature hydrogen-separatingdevice 18, which is operatively coupled to second reformer 16. As shownin FIGS. 1 and 2, high temperature hydrogen-separating device 18 is partof second reformer 16. As shown in FIG. 3, high temperaturehydrogen-separating device 18 is separate from second reformer 16 and isoperatively coupled to second reformer via line 122.

High temperature hydrogen-separating device 18 may include one or morehigh temperature tubular hydrogen-separation membranes 68. Membranes 68may be located in the reforming zone 108 of second reformer 16 andpositioned so that the feed and the reformed product gas may contact themembranes 68. Hydrogen may pass through membrane wall (not shown) ofmembranes 68 to hydrogen conduit 124 located within membranes 68. Themembrane wall of each respective membrane separates hydrogen conduit 124from gaseous communication with non-hydrogen compounds of the reformedproduct gas, feed, and steam in reforming zone 108 of second reformer16. The membrane wall is selectively permeable to hydrogen, elementaland/or molecular, so that hydrogen in reforming zone 108 may passthrough the membrane wall of membrane 68 to hydrogen conduit 124 whileother gases in the reforming zone are prevented from passing to thehydrogen conduit by the membrane wall. Hydrogen flux across hightemperature hydrogen-separating device 18 may be increased or decreasedby adjusting the pressure in second reformer 16. The pressure in secondreformer 16 may be controlled by the rate at which the anode exhauststream is fed to first reformer 14.

Referring to FIG. 3, feed from second reformer 16 is fed to hightemperature hydrogen-separating device 18 via line 122. High temperaturehydrogen-separation device 18 may comprise a member that is selectivelypermeable to hydrogen, either in molecular or elemental form. In apreferred embodiment, the high temperature hydrogen-separation devicecomprises a membrane that is selectively permeable to hydrogen. In anembodiment, the high temperature hydrogen-separation device comprises atubular membrane coated with palladium or a palladium alloy that isselectively permeable to hydrogen.

The gas stream that enters high temperature hydrogen-separation device18 via line 122 may include hydrogen, carbon oxides, and hydrocarbons.The gas stream may contact tubular hydrogen-separation membrane(s) 68and hydrogen may pass through a membrane wall to hydrogen conduit 124located within membranes 68. The membrane wall separates hydrogenconduit 124 from gaseous communication with non-hydrogen compounds, andis selectively permeable to hydrogen, elemental and/or molecular, sothat hydrogen in the entering gas may pass through the membrane wall tohydrogen conduit 124 while other gases are prevented by the membranewall from passing to the hydrogen conduit.

High temperature tubular hydrogen-separation membrane(s) 68 in FIGS. 1and 2 may include a support coated with a thin layer of a metal or alloythat is selectively permeable to hydrogen. The support may be formed ofa ceramic or metallic material that is porous to hydrogen. Porousstainless steel or porous alumina is preferred materials for the supportof the membrane 68. The hydrogen selective metal or alloy coated on thesupport may be selected from metals of Group VIII, including, but notlimited to Pd, Pt, Ni, Ag, Ta, V, Y, Nb, Ce, In, Ho, La, Au, and Ru,particularly in the form of alloys. Palladium and platinum alloys arepreferred. A particularly preferred membrane 68 used in the presentprocess has a very thin film of a palladium alloy having a high surfacearea coating a porous stainless steel support. Membranes of this typecan be prepared using the methods disclosed in U.S. Pat. No. 6,152,987.Thin films of platinum or platinum alloys having a high surface areawould also be suitable as the hydrogen selective material.

Pressure within reforming zone 108 of second reformer 16 is maintainedat a level significantly above the pressure within the hydrogen conduit124 of tubular membrane 68 so that hydrogen is forced through themembrane wall from reforming zone 108 of second reformer 16 intohydrogen conduit 124. In an embodiment, hydrogen conduit 124 ismaintained at or near atmospheric pressure, and the reforming zone 108is maintained at a pressure of at least 0.5 MPa, or at least 1.0 MPa, orat least 2 MPa, or at least 3 MPa. As noted above, reforming zone 108may be maintained at such elevated pressures by compressing the feedfrom first reformer 14 with compressor 94 and injecting the mixture offeed at high-pressure into reforming zone 108. Alternatively, reformingzone 108 may be maintained at such high-pressures by mixinghigh-pressure steam with the feed as described above and injecting thehigh-pressure mixture into reforming zone 108 of second reformer 16.Alternatively, the reforming zone 108 may be maintained at suchhigh-pressures by mixing high-pressure steam with the hydrocarbon streamin first reformer 14 or a reforming zone of the first reformer andinjecting a high-pressure feed produced in the first reformer intosecond reformer 16 either directly or through one or more heatexchangers 90. Reforming zone 108 of second reformer 16 may bemaintained at a pressure of at least 0.5 MPa, or at least 1.0 MPa, or atleast 2.0 MPa, or at least 3.0 MPa.

The temperature at which the steam reforming feed, and optionallyadditional steam, is/are mixed and contacted with the reforming catalystin reforming zone 108 of second reformer 16 is at least 400° C., andpreferably may range from 400° C. to 650° C., most preferably in a rangeof from 450° C. to 550° C. Typical steam reformers are run attemperatures of 750° C. or higher to obtain equilibrium conversions thatsufficiently high. In the present process, the reforming reaction isdriven towards the production of hydrogen in the reformer operatingtemperature range of 400° C. to 650° C. by continuous removal ofhydrogen from reforming zone 108 into hydrogen conduit 124 of membranes68, and thence removed from second reformer 16. In this way, the presentprocess may obtain nearly complete conversion of reactants to hydrogenwithout equilibrium limitations. An operating temperature of 400° C. to650° C. favors the shift reaction as well, converting carbon monoxideand steam to more hydrogen, which is then removed from reforming zone108 into hydrogen conduit 124 through the membrane wall of themembrane(s). Nearly complete conversion of hydrocarbons and carbonmonoxide to hydrogen and carbon dioxide by the reforming and water-gasshift reactions may be achieved in second reformer 16 since equilibriumis never reached due to the continuous removal of hydrogen from thesecond reformer.

In an embodiment, the steam reforming feed provided from first reformer14 and/or a reforming zone of the first reformer to the second reformer16 supplies heat to drive the reactions in the second reformer. Thesteam reforming feed produced from first reformer 14 and/or a reformingzone of the first reformer to second reformer 16 may contain sufficientthermal energy to drive the reactions in the second reformer, and mayhave a temperature from 400° C. to 950° C. The thermal energy of thesteam reforming feed produced from first reformer 14 and/or a reformingzone of the first reformer may be in excess of the thermal energy neededto drive the reactions in second reformer 16, and, as described above,the feed may be cooled to a temperature from 400° C. to less than 600°C. in heat exchanger 90 and/or by injecting water into the feed prior tothe feed being fed to second reformer 16. Having a feed at or near thetemperatures required for second reformer 16 may be preferable sothat 1) temperature within second reformer 16 may be adjusted to favorthe production of hydrogen in the water-gas shift reaction; 2)membrane(s) 68 life-span may be extended; and 3) performance ofcompressor 94 is improved. The transfer of thermal energy from firstreformer 14 to second reformer 16 is extremely efficient since thermalenergy from the first reformer is contained in the feed, which isintimately involved in the reactions within the second reformer.

The hydrogen-containing stream, is formed from the reformed product gasin high temperature hydrogen-separation device 18 by selectively passinghydrogen through the membrane wall of hydrogen-separation membrane(s) 68into the hydrogen conduit 124 to separate the hydrogen-containing gasstream from the reformed product gas. The hydrogen-containing gas streammay contain a very high concentration of hydrogen, and may contain atleast 0.9, or at least 0.95, or at least 0.98 mole fraction hydrogen.

The hydrogen-containing gas stream may be separated from the reformedproduct gas at a relatively high rate due to the high flux of hydrogenthrough the hydrogen-separation membrane(s) 68. In an embodiment, thetemperature at which the hydrogen is separated from the reformed productgas through the hydrogen-separation membrane(s) 68 is at least 300° C.,or from about 350° C. to about 600° C., or from 400° C. to 500° C.Hydrogen is passed at a high flux rate through the hydrogen-separationmembrane(s) 68 since hydrogen is present in second reformer 16 at a highpartial pressure. The high partial pressure of hydrogen in secondreformer 16 is due to 1) significant quantities of hydrogen in the anodeexhaust stream fed to the first reformer 14 and passed to the secondreformer in the feed; 2) hydrogen produced in the first reformer and fedto the second reformer; and 3) hydrogen produced in the second reformerby the reforming and shift reactions. No sweep gas is necessary toassist removing hydrogen from hydrogen conduit 124 and out of hightemperature hydrogen-separation device 18 due to the high rate thathydrogen is separated from the reformed product.

As shown in FIGS. 1-2, the hydrogen-containing gas stream exits hightemperature hydrogen-separation device 18 and enters anode 24 of moltencarbonate fuel cell 12 via hydrogen conduit 124 through lines 126 and 34into anode inlet 30. Alternatively, the hydrogen-containing gas is feddirectly to anode inlet 30 via line 126. The hydrogen gas streamprovides hydrogen to anode 24 for electrochemical reaction with anoxidant at one or more anode electrodes along the anode path length infuel cell 12. A partial pressure of the molecular hydrogen enteringsecond reformer 16 is higher than a partial pressure of the molecularhydrogen in the hydrogen-containing gas stream exiting high temperaturehydrogen-separation device 18. The difference in partial pressurebetween second reformer 16 and the partial pressure of the molecularhydrogen in the hydrogen-containing gas stream exiting high temperaturehydrogen-separation device 18 drives the reforming reaction and/orwater-gas shift reactions to make more hydrogen. In some embodiments, asweep gas, for example steam, may be injected into the hydrogen conduitto sweep hydrogen from the inner portion of the membrane wall memberinto the hydrogen conduit, thereby increasing the rate hydrogen may beseparated from the reforming zone by the hydrogen-separation membrane.

Prior to feeding the hydrogen-containing gas stream to the anode 24, thehydrogen-containing gas stream, or a portion thereof, may be fed to heatexchanger 72 to heat the hydrocarbon stream and cool the hydrogen gasstream via line 128. The hydrogen-containing gas stream may have atemperature from 400° C. to 650° C., typically a temperature from 450°C. to 550° C., upon exiting high temperature hydrogen-separation device18. The pressure of the hydrogen-containing gas exiting high temperaturehydrogen-separation device 18 may have a pressure of about 0.1 MPa, orfrom 0.01 MPa to 0.5 MPa, or from 0.02 MPa to 0.4 MPa or from 0.3 to 0.1MPa. In a preferred embodiment, a hydrogen-containing gas stream exitinghigh temperature hydrogen-separation device 18 has a temperature ofabout 450° C. and a pressure of about 0.1 MPa. The pressure andtemperature of the hydrogen-containing gas stream exiting hightemperature hydrogen-separation device 18 may be suitable for directlyfeeding the hydrogen-containing gas stream directly to anode inlet 30 ofmolten carbonate fuel cell 12.

The hydrocarbon stream may optionally be heated by exchanging heat withthe hydrogen gas stream in heat exchanger 72, and optionally byexchanging heat with the carbon dioxide gas stream as described below.The hydrogen gas stream fed to anode 24 of molten carbonate fuel cell 12may be cooled to a temperature of at most 400° C., or at most 300° C.,or at most 200° C., or at most 150° C., or temperatures from 20° C. to400° C., or from 25° C. to 250° C. to control the operating temperatureof the molten carbonate fuel cell within a range from 600° C. to 700°C., in combination with selecting and controlling the temperature of theoxidant-containing gas stream fed to cathode 26 of molten carbonate fuelcell 12. The hydrogen-containing gas stream, or a portion thereof, maytypically be cooled to a temperature from 200° C. to 400° C. byexchanging heat with the hydrocarbon stream in heat exchanger 72.Optionally, the hydrogen gas stream, or a portion thereof, may be cooledfurther by passing the hydrogen gas stream, or the portion thereof, fromheat exchanger 72 to one or more additional heat exchangers (not shown)to exchange further heat with the hydrocarbon stream or with a waterstream in each of the one or more additional heat exchangers. Ifadditional heat exchangers are employed in the system, the hydrogen gasstream, or the portion thereof, may be cooled to a temperature of from20° C. to 200° C., preferably from 25° C. to 100° C. In an embodiment, aportion of the hydrogen gas stream may be cooled in heat exchanger 72and, optionally one or more additional heat exchangers, and a portion ofthe hydrogen gas stream may be fed to anode 24 of molten carbonate fuelcell 12 without being cooled in a heat exchanger, where the combinedportions of the hydrogen gas stream may be fed to the anode of the fuelcell at a temperature of at most 400° C., or at most 300° C., or at most200° C., or at most 150° C., or temperatures from 20° C. to 400° C., orfrom 25° C. to 100° C.

The flow rate of the hydrogen gas stream, or portion thereof, to heatexchangers 72, 22, and, optionally to one or more additional heatexchangers, may be selected and controlled to control the temperature ofthe hydrogen gas stream fed to anode 24 of molten carbonate fuel cell12. The flow rate of the hydrogen gas stream, or a portion thereof, toheat exchanger 22, and the optional additional heat exchanger(s) may beselected and controlled by adjusting metering valves 36, 130, and 132.Metering valves 36 and 130 may be adjusted to control the flow of thehydrogen gas stream, or a portion thereof, to anode 24 of moltencarbonate fuel cell 12 through line 126 without cooling the hydrogen gasstream, or the portion thereof. Metering valve 130 may also control theflow of the hydrogen gas stream, or a portion thereof, to heat exchanger22. Metering valve 132 may be adjusted to control the flow of thehydrogen gas stream, or a portion thereof, to heat exchanger 72 and anyoptional additional heat exchangers through line 128. Metering valves130 and 132 may be adjusted in coordination to provide the desireddegree of cooling to the hydrogen gas stream prior to feeding thehydrogen gas stream to anode 24 of molten carbonate fuel cell 12. In anembodiment, metering valves 130 and 132 may be adjusted in coordinationautomatically in response to feedback measurements of the temperature ofthe anode exhaust stream and/or the cathode exhaust stream exiting fuelcell 12. The hydrogen gas stream provides hydrogen to the anode 24 forelectrochemical reaction with an oxidant at one or more anode electrodesalong the anode path length in fuel cell 12. The rate the hydrogen gasstream is fed to anode 24 of molten carbonate fuel cell 12 may beselected by selecting the rate that the feed is fed to second reformer16, which in turn may be selected by the rate that the hydrocarbonstream is fed to first reformer 14, which may be controlled by adjustingthe hydrocarbon stream inlet valve 106.

Any portion of the hydrogen-containing gas stream fed to heat exchanger72, and optionally the additional heat exchanger(s), may be fed from theheat exchanger, or through the last additional heat exchanger used tocool the hydrogen-containing gas stream with any portion of the hydrogengas stream routed around the heat exchangers to the anode of the moltencarbonate fuel cell. In an embodiment, the combined portions of thehydrogen-containing gas stream or the hydrogen-containing gas streamexiting high temperature hydrogen-separation device 18 may be compressedin a compressor (not shown) to increase the pressure of the hydrogen gasstream, and then the hydrogen gas stream may be fed to the anode. In anembodiment, the hydrogen gas stream may be compressed to a pressure from0.15 MPa to 0.5 MPa, or from 0.2 MPa to 0.3 MPa, or up to 0.7 MPa, or upto 1 MPa. All or part of the energy required to drive the compressor maybe provided by expansion of a high-pressure carbon dioxide stream,formed as described below, and/or the high-pressure steam through one ormore turbines.

Alternatively, the rate that the hydrogen gas stream is fed to anode 24of molten carbonate fuel cell 12 may be selected by controlling meteringvalves 36 and 134 in a coordinated manner. Metering valve 36 may beadjusted to increase or decrease the flow of the hydrogen gas streaminto anode 24. Metering valve 134 may be adjusted to increase ordecrease flow of the hydrogen gas stream to hydrogen source 64. Meteringvalves 36 and 134 may be controlled in a coordinated manner so that aselected rate of the hydrogen gas stream may be fed to anode 24 ofmolten carbonate fuel cell 12 through line 34 while a portion of thehydrogen gas stream in excess of the amount of hydrogen gas streamrequired to provide the selected rate may be fed to the hydrogen source64 through line 136.

A hydrogen-depleted reformed product gas stream may be removed from hightemperature hydrogen-separation device 18 via line 48, where thehydrogen-depleted reformed product gas stream may include unreacted feedand gaseous non-hydrogen reformed products in the reformed product gas.The non-hydrogen reformed products and unreacted feed may include carbondioxide, water (as steam), and small amounts of carbon monoxide andunreacted hydrocarbons. Small amounts of hydrogen may also be containedin the hydrogen-depleted reformed product gas stream as well.

In an embodiment, the hydrogen-depleted reformed product gas streamexiting high temperature hydrogen-separation device 18 may be a carbondioxide gas stream containing at least 0.8, or at least 0.9, or at least0.95, or at least 0.98 mole fraction carbon dioxide on a dry basis. Thecarbon dioxide gas stream is a high-pressure gas stream, having apressure of at least 0.5 MPa, or at least 1 MPa, or at least 2 MPa, orat least 2.5 MPa. Hereafter, the hydrogen-depleted reformed product gasstream will be referred to as the high-pressure carbon dioxide gasstream. The temperature of the high-pressure carbon dioxide gas streamexiting hydrogen-separation device 18 is at least 400° C. or typicallybetween 425° C. and 600° C. or between 450° C. and 550° C.

The high-pressure carbon dioxide gas stream may exit high temperaturehydrogen-separation device 18 and be fed to cathode 26 of fuel cell 12via lines 48 and 44. As shown, the high-pressure carbon dioxide gasstream passes through heat exchanger 22 and may be utilized to heat theoxidant gas stream. In an embodiment, a portion of the carbon dioxidestream is mixed directly with the oxidant gas stream entering cathode 26via line 44.

In a preferred embodiment, the high-pressure carbon dioxide gas streamis fed to catalytic partial oxidation reformer 20 via line 48. Incatalytic partial oxidation reformer 20, residual hydrocarbons (forexample, methane, ethane, and propane) in the carbon dioxide stream arecombusted in the presence of oxygen or air fed from oxidant source 42via line 56 to form a hot effluent combustion stream that passes throughheat exchanger 22 via line 138 and is fed to cathode 26 via line 44. Inan embodiment, the combustion stream is fed directly to cathode 26 vialines 138 and 44. An amount of molecular oxygen in theoxidant-containing stream fed to catalytic partial oxidation reformer 20is at least 0.9 times but not more than 1.1 times the stoichiometricamount required for complete combustion of hydrocarbons in the carbondioxide stream.

Hot combustion stream may include a substantial amount of carbondioxide, but may also include nitrogen gas and water. The hot combustionstream exiting catalytic partial oxidation reformer 20 may have atemperature ranging from at least 750° C. to 1050° C., or from 800° C.to 1000° C., or from 850° C. to 900° C. Heat from the hot combustion gasmay be exchanged with hydrogen-containing gas stream in heat exchanger22 and/or oxidant-containing gas stream in the heat exchanger. As shownin FIG. 2, at least a portion of the heat from the combustion streamexiting catalytic partial oxidation reforming 20 may be exchanged withfirst reformer 14 in heat exchanger 98 via line 96.

In an embodiment, hot combustion gas may be fed directly to cathodeexhaust inlet 38. A temperature of the oxidant-containing gas may beadjusted so that a temperature of the cathode exhaust stream exiting thefuel cell ranges from 550° C. to 700° C. The oxidant-containing gastemperature may be adjusted to a temperature from 150° C. to 450° C.through cooling and/or heating in heat exchanger 22. Flow of theoxidant-containing gas stream from high temperature hydrogen-separationdevice 18 to heat exchanger 22 and/or catalytic partial oxidationreformer 20 may be controlled by adjusting metering valves 46, 58, and140.

The hot combustion gas stream may contain significant amounts of wateras steam as it exits catalytic partial oxidation reforming 20. In anembodiment, the steam may be removed from the hot combustion gas streamby cooling the hot combustion gas stream in heat exchanger 22 and/or inheat exchanger 72 and, if necessary, one or more additional heatexchangers (not shown) and condensing water from the stream.

The high-pressure carbon dioxide gas stream from high temperaturehydrogen-separation device 18 may be utilized to heat the hydrocarbonstream by passing the carbon dioxide containing gas stream through line142 to heat exchanger 72 while feeding the hydrocarbon stream into theheat exchanger 72 through the hydrocarbon stream line 62. Flow of thehigh-pressure carbon dioxide stream from high temperaturehydrogen-separation device 18 to heat exchanger 72 may be controlled byadjusting metering valve 144. Metering valve 144 may be adjusted tocontrol the flow of the carbon dioxide stream to heat exchanger 72 toheat the hydrocarbon stream to a selected temperature. The hydrocarbonstream may be heated to a temperature such that the hydrocarbon streamhas a temperature of at least 150° C., or from 200° C. to 500° C. as thehydrocarbon stream is fed to first reformer 14.

Metering valves 46, 58, and 140, may be adjusted automatically by afeedback mechanism, where the feedback mechanism may measure thetemperature of the cathode exhaust stream exiting fuel cell 12 and/orthe temperature of the hydrocarbon stream entering first reformer 14 andadjust metering valves 46, 58, and 140 to maintain the temperature ofthe cathode exhaust stream and/or the hydrocarbon stream entering firstreformer 14 within set limits while maintaining the internal pressurewithin second reformer 16 and/or high temperature hydrogen-separationdevice 18 at a desired level.

The hydrogen gas stream and the oxidant (carbonate)—generated by thereaction of oxygen and carbon dioxide at the cathode—are preferablymixed at the one or more anode electrodes of the fuel cell 12 asdescribed above to generate electricity at an electrical power densityof at least 0.1 W/cm², more preferably at least 0.15 W/cm², or at least0.2 W/cm², or at least 0.3 W/cm². Electricity may be generated at suchelectrical power densities by selecting and controlling the rate thatthe hydrogen gas stream is fed to anode 24 of fuel cell 12 and the ratethat the oxidant-containing gas stream is fed to cathode 26 of fuel cell12. The flow rate of the oxidant-containing gas stream to cathode 26 offuel cell 12 may be selected and controlled by adjusting oxidant gasinlet valve 46.

As described above, the flow rate of the hydrogen gas stream to anode 24of fuel cell 12 may be selected and controlled by selecting andcontrolling the rate that the feed is fed to second reformer 16, whichin turn may be selected and controlled by the rate that the hydrocarbonstream is fed to first reformer 14, which may be selected and controlledby adjusting hydrocarbon stream inlet valve 106. Alternatively, asdescribed above, the rate that the hydrogen gas stream is fed to anode24 of fuel cell 12 may be selected and controlled by controllingmetering valves 36, 130, 132, and 134 in a coordinated manner. In anembodiment, metering valves 36, 130, 132, and 134 may be automaticallyadjusted by a feedback mechanism to maintain a selected flow rate of thehydrogen gas stream to anode 24, where the feedback mechanism mayoperate based upon measurements of hydrogen content in the anode exhauststream, or water content in the anode exhaust stream, or the ratio ofwater formed in the fuel cell relative to hydrogen in the anode exhauststream.

In the process of the invention, mixing the hydrogen gas stream and theoxidant at the one or more anode electrodes generates water (as steam)by the oxidation of a portion of hydrogen present in the hydrogen gasstream fed to fuel cell 12 with the oxidant. Water generated by theoxidation of hydrogen with an oxidant is swept through anode 24 of fuelcell 12 by the unreacted portion of the hydrogen gas stream to exitanode 24 as part of the anode exhaust stream.

In an embodiment of the process of the invention, the flow rate that thehydrogen gas stream is fed to anode 24 may be selected and controlled sothe ratio of amount of water formed in fuel cell 12 per unit of time tothe amount of hydrogen in the anode exhaust per unit of time is at most1.0, or at most 0.75, or at most 0.67, or at most 0.43, or at most 0.25,or at most 0.11. In an embodiment, the amount of water formed in fuelcell 12 and the amount of hydrogen in the anode exhaust may be measuredin moles so that the ratio of the amount of water formed in the fuelcell per unit of time to the amount of hydrogen in the anode exhaust perunit of time in moles per unit of time is at most 1.0, or at most 0.75,or at most 0.67, or at most 0.43, or at most 0.25, or at most 0.11. Inan embodiment, the flow rate that the hydrogen gas stream is fed toanode 24 may be selected and controlled so the per pass hydrogenutilization rate in fuel cell 12 is less than 50%, or at most 45%, or atmost 40%, or at most 30%, or at most 20%, or at most 10%.

In another embodiment of the process of the invention, the flow ratethat the hydrogen gas stream is fed to anode 24 may be selected andcontrolled so the anode exhaust stream contains at least 0.6, or atleast 0.7, or at least 0.8, or at least 0.9 mole fraction hydrogen. Inan another embodiment, the flow rate that the hydrogen gas stream is fedto anode 24 may be selected and controlled so the anode exhaust streamcontains greater than 50%, or at least 60%, or at least 70%, or at least80%, or at least 90% of the hydrogen in the hydrogen gas stream fed toanode 24.

In some embodiments, the flow rate that the carbon dioxide stream is fedto cathode 26 may be selected and controlled so that the partialpressure of the carbon dioxide in majority of the cathode portion of themolten carbonate fuel cell is higher than a partial pressure of carbondioxide in a majority of an anode portion of the molten carbonate fuelcell. In an embodiment, the flow rate that the carbon dioxide stream isfed to cathode 26 may be selected and controlled so that the partialpressure of the carbon dioxide in the cathode exhaust stream exiting thefuel cell is greater than the partial pressure of the carbon dioxide inthe anode exhaust stream exiting the fuel cell. The flow rate of carbondioxide is selected and controlled so that the partial pressure ofcarbon dioxide in at least 75 percent, or at least 95 percent, orsubstantially all of the cathode portion of the molten carbonate fuelcells is higher than a partial pressure of carbon dioxide in at least 75percent, 95 percent or substantially all of the anode portion of themolten carbonate fuel cell.

Operating the molten carbonate fuel cell to control the ΔP_(CO2) atpressures at above 0, at any concentration of air and/or any utilizationof hydrogen, may inhibit carbon dioxide starvation of the moltencarbonate fuel cell and enhance the cell potential of the moltencarbonate fuel cell. The flow rate that the carbon dioxide stream is fedto cathode 26 of molten carbonate fuel cell 12 may be selected andcontrolled such that the delta in the carbon dioxide partial pressures,as determined by the equation: (ΔP_(CO2))=(P_(CO2) ^(c)) (P_(CO2) ^(a)),is about or above 0 bara, from 0.01 to 0.2 bara, or from 0.05 to 0.15bara, when the hydrogen utilization is at most 60%, at most 50% or atmost 40%, at most 30%, at most 20%, or at most 10% and/or the flow ofair is controlled such that a molar ratio of carbon dioxide to molecularoxygen is about 2.

EXAMPLES

Non-restrictive examples are set forth below.

A UniSim® simulation program (Honeywell) in combination withcalculations for cell potential was used to construct a detailed processsimulation for the molten carbonate fuel cell systems of the presentinvention. The UniSim program was used to obtain material balance andenergy balance data. The detailed process simulation was repeatedlysolved for the different values of hydrogen utilization, and otherrelevant system parameters. The detailed process simulation outputincluded detailed composition data for all process streams entering andexiting the molten carbonate fuel cell. For high temperature fuel cells,activation losses are small and the cell potential may be obtained overthe practical range of current densities by considering only ohmic andelectrode losses. As such, the cell potential (V) of a molten carbonatefuel cell is the difference between the open circuit voltage (E) and thelosses (iR) as shown in Equation (1).

V=E−iR  (1)

where V and E have units of volts or millivolts, i is the currentdensity (mA/cm²) and R (Ωcm²) is the combination of Ohmic (R_(ohm)),cathode (□_(c)) and anode (□_(a)) resistance, combining electrolyte,cathode and anode together as shown in Equation (2).

R=R _(ohm)+□_(c)+□_(a)  (2)

E is obtained from the Nernst equation:

E=E°+(RT/2F)ln(P _(H2) P _(O2) ^(0.5) /P _(H2O))+(RT/2F)ln(P _(CO2) ^(c)/P _(CO2) ^(a))  (3)

Examples 1

The detailed process simulations described above was used to simulatecell voltage versus current density and power density formation formolten carbonate fuel cell systems described herein where the firstreformer was heated by the anode exhaust, with no other heating. Forexample, systems depicted by FIG. 1. The heat for the second reformerwas heated by exchange with the hot effluent from the catalytic partialoxidation reformer. The output temperature of the effluent from thecatalytic partial oxidation reformer was increased by using the cathodeexhaust to preheat the catalytic oxidation reformer air feed.

Example 2

The simulation described above was used to simulate cell voltage versuscurrent density and power density formation for molten carbonate fuelcell systems described herein where the first reformer is heated byanode exhaust and heat from a catalytic partial oxidation reformer. Forexample, systems depicted in FIG. 2.

For Examples 1 and 2, the molten carbonate fuel cell was operated at apressure of 1 bara (about 0.1 MPa or about 1 atm) and a temperature of650° C. The flow of feed to the cathode of the molten carbonate fuelcell was counter current to the flow of feed to the anode. Air was usedas the source of oxygen. Values for air were used to produce a molarratio of carbon dioxide to molecular oxygen of 2 at various hydrogenutilizations. The percent hydrogen utilization for the molten carbonatefuel cell, operating conditions of the first and second reformer, steamto carbon ratios, and percent conversion of benzene to hydrogen forExample 1 and 2 simulations are listed in TABLE 1. R in Equation 2 wasobtained from J. Power Sources 2002, 112, pp. 509-518 and assumed to beequal to 0.75 Ωcm², as

The data from for Examples 1 and 2 simulations were compared toliterature values for cell voltage, current density, and power densityof state of the art molten carbonate fuel cells described by Larmine etal., in “Fuel Cell Systems Explained,” 2003, Wiley & Sons, page 199.

TABLE 1 Conversion Temp., Temp., Pressure, Steam/Carbon Steam/Carbon ofH₂ 1^(st) 2^(nd) 2^(nd) Ratio, Ratio, Benzene Utilization Reformer,Reformer, Reformer, 1^(st) 2^(nd) to Hydrogen % ° C. ° C. bara ReformerReformer % 20 619 500 15 2.5 3 94 30 591 500 15 2.5 3 95 40 569 500 152.5 3 96 50 551 500 15 2.5 3 96 60 536 500 15 2.5 3 97

FIG. 4 depicts cell voltage (mV) versus current density (mA/cm²) for themolten carbonate fuel cell systems simulated in Examples 1 and 2 andliterature values for a molten carbonate fuel cell having reformate as afeed. The molten carbonate fuel cells were operated at a hydrogenutilization of 20% and 30%. Data line 160 depicts cell voltage (mV)versus current density (mA/cm²) at a hydrogen utilization of 20% for amolten carbonate fuel cell system for Examples 1 and 2. Data line 162depicts cell voltage (mV) versus current density (mA/cm²) at a hydrogenutilization of 30% for Examples 1 and 2. Data line 164 depicts cellvoltage (mV) versus current density (mA/cm²) for state of the art moltencarbonate fuel cell systems as described by Larmine et al., in “FuelCell Systems Explained,” 2003, Wiley & Sons, page 199. As shown in FIG.4, for a given current density, the cell voltage of the molten carbonatefuel cell systems described herein are higher than the cell voltage ofstate of the art molten carbonate fuel cell having reformate gas as afeed.

FIG. 5 depicts power density (W/cm²) vs. current density (mA/cm²) forthe molten carbonate fuel cell system simulated in Examples 1 and 2operated at a hydrogen utilization of 20% and 30%, and literature valuesfor a molten carbonate fuel cell having a reformate gas as a feed. Dataline 166 depicts power density (W/cm²) vs. current density (mA/cm²) at ahydrogen utilization of 20% for Examples 1 and 2. Data line 168 depictspower density (W/cm²) vs. current density (mA/cm²) at a hydrogenutilization of 30% for Examples 1 and 2. Data line 170 depicts powerdensity (W/cm²) vs. current density (mA/cm²) for state of the art moltencarbonate fuel cell systems as described by Larmine et al., in “FuelCell Systems Explained, 2003, Wiley & Sons, page 199. As shown in FIG.5, for a given current density, the power density of the moltencarbonate fuel cell systems described herein are higher than the powerdensity of the molten carbonate fuel cell having reformate gas as afeed.

FIG. 6 depicts excess carbon dioxide (ΔP_(CO2) (bara)), and total fuelcell potential (mV) versus hydrogen utilization for Example 1. Data line172 represents excess carbon dioxide values (at a given hydrogenutilization and a current density of 200 mA/cm². Data 174 representsaverage excess carbon dioxide values at a given hydrogen utilization.Data line 176 represents total cell potential (mV) at given hydrogenutilization as determined from the Nernst equation for the fuel cell. Asshown in FIG. 6, ΔP_(CO2) decreases and cell potential decreases ashydrogen utilization increase, thus operating the molten carbonate fuelsystem at a hydrogen utilization of less than 50% with carbon dioxideflooding results enhanced cell potential for the molten carbonate fuelcell.

FIG. 7 depicts the carbon dioxide portion of the fuel cell potential(mV) of FIG. 6. Data line 178 represents the carbon dioxide portion ofthe cell potential (mV) of the fuel cell (for example, the(RT/2F)ln(P_(CO2) ^(c)/P_(CO2) ^(a)) portion of the Nernst equation). Asshown in FIG. 7, a cell voltage of the fuel cell is boosted when thecathode portion of the fuel cell is flooded with carbon dioxide. Forexample, at a 20% utilization of hydrogen and operating the fuel cellwith an excess carbon dioxide value of about 0.105, 30 mV of the totalfuel cell potential was attributed to the excess carbon dioxide.

As shown in FIGS. 6 and 7, cell potential is maximized when the amountof carbon dioxide provided to the fuel cell is in excess (ΔP_(CO2)>0)and the percent hydrogen utilization is low (for example, less than 35%,less than 30%, or less than 20%). Thus, operating the molten carbonatefuel system at a hydrogen utilization of less than 50% and providing anexcess of carbon dioxide to a cathode portion of the molten carbonatefuel cell such that a partial pressure of carbon dioxide in a majorityof the cathode portion of the molten carbonate fuel cell is higher thana partial pressure of carbon dioxide in a majority of an anode portionof the molten carbonate fuel cell enhances cell potential and, therebyenhances cell voltage of the molten carbonate fuel cell.

Example 3

The simulations described above were used to determine the currentdensity, cell voltage, and power density for a molten carbonate fuelcell operated at 7 bara (about 0.7 MPa or about 7 atm) for a moltencarbonate fuel cell system that includes the first reformer heated byanode exhaust (for example, the system depicted in FIG. 1). The moltencarbonate fuel cell was operated at a pressure of 7 bara and atemperature of 650° C. at a hydrogen utilization of 20% or 30%. Thefirst reformer had a steam to carbon ratio of 2.5. The temperature ofthe first reformer was allowed to be varied. The second reformer, incombination with the high temperature hydrogen-separation device, had atemperature of 500° C. and a pressure of 15 bara. Air was used as thesource of oxygen. Values for air were used so that the ratio of carbondioxide to molecular oxygen in the cathode feed was stoichiometric, thusminimizing cathode side concentration polarization. In all cases, thecombined carbon conversion values for the system using benzene as thefeed was between 93% and 95%. Heat of reaction for the second reformerwas supplied by heat integration within the system. R was calculated bycalculating the individual terms in Equation 2 above separately by themethod described by C. Y. Yuh and J. R. Selman, in J. Electrochem. Soc.,Vol. 138, No. 12, December 1991. For Example, 3, R was calculated to be0.57 Ω·cm².

FIG. 8 depicts cell voltage (mV) versus current density (mA/cm²) for amolten carbonate fuel cell as depicted in FIG. 1. Data line 180 depictscell voltage (mV) versus current density (mA/cm²) at a hydrogenutilization of 20%. Data line 182 depicts cell voltage (mV) versuscurrent density (mA/cm²) at a hydrogen utilization of 30%. ComparingFIG. 4 with FIG. 8, at a given current density, a higher cell voltage isobserved for molten carbonate fuel cell systems operated at pressures ofabout 7 bara as compared to the cell voltages for molten carbonate fuelcell systems operated at 1 bara.

FIG. 9 depicts power density (W/cm²) versus current density for a moltencarbonate fuel cell system as depicted in FIG. 1 and a state of themolten carbonate fuel cell. Data line 184 depicts power density (W/cm²)versus current density (mA/cm²) at a hydrogen utilization of 20%. Dataline 186 depicts power density (W/cm²) versus current density (mA/cm²)at a hydrogen utilization of 30%. Data point 188 depicts power density(W/cm²) versus current density (mA/cm²) for a state of the art moltencarbonate fuel cell system as described by J. R. Selman in Journal ofPower Sources, 2006, pp. 852-857. As shown in FIG. 9, at a currentdensity of about 300 mA/cm², the power density of the molten carbonatefuel cell systems described herein are higher than the power density ofthe state of the art molten carbonate fuel cell.

Example 4

The simulation described above was used to compare the use of methane tobenzene as fuel sources for molten carbonate fuel cell systems systemwhere the first reformer is heated by the anode exhaust, with no otherheating. For example, the system depicted in FIG. 1. Heat of reactionfor the second reformer was supplied by heat integration within thesystem. For these simulations, the molten carbonate fuel cell wasoperated at a pressure of 1 bara (about 0.1 MPa or about 1 atm) and atemperature of 650° C. Air was used as the source of oxygen. Values forair were used to produce a molar ratio of carbon dioxide to molecularoxygen of 2 at various hydrogen utilizations. The amount of fuel feed tothe first reformer was 100 kgmol/hr for benzene and 600 kgmol/hr formethane. The percent hydrogen utilization for the molten carbonate fuelcell, operating conditions of the first and second reformer, and steamto carbon ratios are listed in TABLE 2 for benzene and TABLE 3 formethane. R in Equation 2 was assumed to be equal to 0.75 Ω·cm², asobtained from J. Power Sources 2002, 112, pp. 509-518.

TABLE 2 Temp., Temp., Pressure, Steam/ Steam/ H₂ 1^(st) 2nd 2nd CarbonCarbon Utilization, Reformer, Reformer, Reformer, Ratio, 1^(st) Ratio,2^(nd) % ° C. ° C. bara Reformer Reformer 20 605 500 15 3.0 3 30 574 50015 3.2 3 40 549 500 15 3.3 3 50 527 500 15 3.3 3

TABLE 3 Temp., Temp., Pressure, Steam/ Steam/ H₂ 1^(st) 2nd 2nd CarbonCarbon Utilization, Reformer, Reformer, Reformer, Ratio, 1^(st) Ratio,2^(nd) % ° C. ° C. bara Reformer Reformer 20 624 500 15 1.9 3 30 596 50015 2.0 3 40 574 500 15 2.1 3 50 555 500 15 2.1 3

FIG. 10 depicts cell voltage (mV) versus current density (mA/cm²) formolten carbonate fuel cell systems using benzene or methane as a fuelsource. Data line 190 depicts cell voltage (mV) versus current density(mA/cm²) at a hydrogen utilization of 20% using benzene as a feedsource. Data line 192 depicts cell voltage (mV) versus current density(mA/cm²) at a hydrogen utilization of 20% using methane as a feedsource. As shown in FIG. 10, at all current densities, a higher cellvoltage is observed for molten carbonate fuel cell systems when benzeneis used as a fuel source for the first reformer.

FIG. 11 depicts average excess carbon dioxide (ΔP_(CO2(avg))) versuspercent hydrogen utilization for a molten carbonate fuel cell systemsusing benzene or methane as a fuel source at a current density of 200mA/cm². Data line 194 represents average excess carbon dioxide values ata given hydrogen utilization for benzene. Data 196 represents averageexcess carbon dioxide values for methane. As shown in FIG. 11, athydrogen utilizations of less than 50%, benzene provides more excesscarbon dioxide at a given hydrogen utilization than methane. Thus, moremoles of carbon dioxide per mole of hydrogen is produced when benzene isused as a fuel source.

As shown in Examples 1-4, the molten carbonate fuel cell systems andprocesses described herein provide enhanced current density, currentvoltage, power density and inhibit carbon dioxide starvation of the fuelcell by providing a hydrogen-containing stream comprising molecularhydrogen to an anode portion of a molten carbonate fuel cell;controlling a flow rate of the hydrogen-containing stream to the anodesuch that molecular hydrogen utilization in the anode is less than 50%;mixing anode exhaust comprising molecular hydrogen from the moltencarbonate fuel cell with a hydrocarbon stream comprising hydrocarbons,wherein the anode exhaust mixed with the hydrocarbon stream has atemperature from 500° C. to 700° C.; contacting at least a portion ofthe mixture of anode exhaust and the hydrocarbon stream with a catalystto produce a steam reforming feed comprising one or more gaseoushydrocarbons, molecular hydrogen, and at least one carbon oxide;separating at least a portion of the molecular hydrogen from the steamreforming feed; and providing at least a portion of the separatedmolecular hydrogen to the molten carbonate fuel cell anode as at least aportion of the hydrogen-containing stream comprising molecular hydrogen.

We claim:
 1. A molten carbonate system, comprising: a molten carbonate fuel cell comprising an anode portion and a cathode portion, said molten carbonate fuel cell configured to receive a hydrogen-containing stream comprising molecular hydrogen at a flow rate such that hydrogen utilization in an anode of the molten carbonate fuel cell is less than 50%; one or more reformers operatively coupled to the molten carbonate fuel cell, at least one reformer being configured to receive anode exhaust from the molten carbonate fuel cell and hydrocarbons, and being configured to allow the anode exhaust to sufficiently mix with a stream comprising hydrocarbons to at least partially reform some of the hydrocarbons to produce a reformed product stream, wherein the reformed product stream comprises molecular hydrogen and at least one carbon oxide; and a high temperature hydrogen-separation device that is part of, or coupled to, at least one of the reformers and operatively coupled to the molten carbonate fuel cell, wherein the high temperature hydrogen-separation device is configured to receive a reformed product stream and to provide a stream comprising at least a portion of the molecular hydrogen to the anode portion of the molten carbonate fuel cell.
 2. The molten carbonate system of claim 1, comprising at least two reformers including, a first reformer configured to receive anode exhaust, and a second reformer configured to receive product from the first reformer and a stream comprising hydrocarbons.
 3. The molten carbonate system of claim 2, wherein the high temperature hydrogen-separation device is operatively coupled to the second reformer.
 4. The molten carbonate system of claim 3, further comprising an oxidizing unit.
 5. The molten carbonate system of claim 3, wherein the high temperature hydrogen-separation device comprises one or more high temperature hydrogen-separating membranes.
 6. The molten carbonate system of claim 4, wherein the oxidizing unit is a catalytic partial oxidation reformer.
 7. The molten carbonate system of claim 6, wherein the second reformer includes a reforming zone, a high temperature hydrogen-separation device and a catalytic partial oxidation reformer.
 8. The molten carbonate system of claim 1, wherein at least a portion of the carbon dioxide provided to the cathode portion of the molten carbonate fuel cell is provided by the high temperature hydrogen-separation device.
 9. The molten carbonate system of claim 1, wherein the molten carbonate fuel cell is operated at a pressure of 0.1 MPa or less.
 10. The molten carbonate system of claim 2, where at least some of the hydrocarbons of the hydrocarbon stream comprise one or more vaporizable hydrocarbons having a carbon number of at least
 4. 11. The molten carbonate system of claim 1, wherein the hydrogen-containing stream comprises at least 0.6 mol fraction molecular hydrogen.
 12. The molten carbonate system of claim 1, further comprising providing air and carbon dioxide to the cathode portion of the molten carbonate fuel cell, wherein air comprises molecular oxygen, and the flow rate of air and carbon dioxide are controlled such that the molar ratio of carbon dioxide to molecular oxygen is at least
 2. 13. The molten carbonate system of claim 1, wherein electricity is generated from the molten carbonate fuel cell at an electrical power density of at least 0.1 W/cm² at 1 bara.
 14. The molten carbonate system of claim 13, wherein the flow rate of air and carbon dioxide are controlled such that the molar ratio of carbon dioxide to molecular oxygen is at least 2.5.
 15. The molten carbonate system of claim 1, wherein the anode exhaust that is mixed with the stream comprising hydrocarbons has a temperature from 500° C. to 700° C.;
 16. The molten carbonate system of claim 14, wherein electricity is generated from the molten carbonate fuel cell at an electrical power density of at least 0.3 W/cm² at 1 bara. 